Add support for an in-memory routing table cache in order to improve performance when the routing table is retained in a file.  The cache holds the most recently used routing table entries and so can eliminate some file access.

Squashed commit of the following:

    net/route:  Flush in cache when any entry is deleted from the routing table.  When a router matching an IP address is found, add the routing table entry to the cache.

    net/route:  Add utility functions to manage an in-memory cache to improve performance when use a file-based routing table.

This commit adds support for routing tables in files in a file system.  This might be useful for customized, per-unit routing tables.

There are two issues with it however:

1. Reading from file system on a per packet basis could be slow.  I think it probably should have a small, in-memory cache of most frequently used routes for good problem.

2. Currently the delroute logic is disabled due to a problem with the design.  NuttX does not currently support truncate().  Therefore, it is not possible to delete entries from the routing table file.

In this current implementation, that leaves the last entry intact at the end of the file.  An alternative design might include a tag on each record to indicate if the record is valid or not.  That would work but would add complexity to the other routing table functions.

The existing implementation is available for testing purposes only if CONFIG_EXPERIMENTAL=y.

Squashed commit of the following:

    net/route:  The current delroute design depends on file truncation.  However, NuttX does not currently support truncate.  Alternative, more complex designs are possible but not implemented.  At present the file routing table are otherwise functional but the delroute logic is disabled via CONFIG_EXPERIMENTAL.  Enable it only if you plan to fix it.

    net/route:  Fix some issues with locking routing table files.

    net/route:  Add partial implementation of delroute for the case where the routing table is in a file.

    net/route:  Add support for seeking to positions in the routing table.

    net/route: Add net_addroute_ipv4/6() using a file-based routing table.

    net/route: Add net_foreach_ipv4/4() using a file-based routing table.

    net/route:  Initial build, configuration, and fs utilies to support routing tables in a file.

Addes support for read-only routing tables.  Prior to this change, routing tables were only support in RAM and had to be initialized with explicit logic to add the necessary routes to the routing table.  With this change, routes may be defined in the pre-initialized, read-only routing table provided by the board-specific logic

This would be particularly useful, for example, in the case where there is only a single network adaptor and you want all output packets to go to the single adaptor in all cases.  So for that behavior you could add a read-only routing table to the board-specific long that contains a single entry, the default route: 0.0.0.0/0.

Squashed commit of the following:

    net/route:  RAM and ROM routing tables build correctly in all IPv4 and IPv6 configurations.

    net/route: Verify IPv6 ROM route build; Make number of ROM routes a variable, not a configuration item.

    net/route:  Add initial support for ROM-base, read-only routing tables.

    net/route:  Adjust and generalize some structures, rename some functions, and add configuration and build support that will eventually support read-only routing tables.

    net/route:  Some initial though experiments on use of a fixe, read-only routing table

    net/socket/recvfrom.c: Check fromlen integrity before using it.

    net/socket/net_sockets.c: Always check for valid psock before using.

    net/tcp/tcp_send_unbuffered.c:  Avoid using psock beforing checking its integrity.

    sched/timer/timer_create.c: Fix watchdog resource leak if cannot allocate a new timer.

From: Sakari Kapanen <sakari.kapanen@optofidelity.com>
Date: Sun, 24 Sep 2017 13:19:25 +0300
Subject: [PATCH] ICMPv6: fix router advertisement

Fixes several errors preventing icmpv6_radvertise.c from being compiled. Fixes conversions to network byte order (namely vlifetime, plifetime, mtu).

IPv6 source address is set to link-local IP address instead of the address in the netdev structure. This is in compliance to RFC 4861. RA didn't work on Linux before this change.

Finally, router prefix and prefix length are derived from the IPv6 address and netmask in the netdev structure. This seems to make more sense than using a predefined, separate prefix from the config.

In some cases, packets are still not sent behind the router. I found that NuttX sends the ARP requests not to the router but to the target. Mistake in file net/route/netdev_router.c.  From Aleksandr Kazantsev

Networking: drivers/net/loopback.c: Eliminate a warning.  net/netdev/netdev_ifconfig.c: Was not returning all of the address info.

    Networking:  Add implementation of logic for SIOCGIFCONF and SIOCGLIFCOF IOCTL commnds.
    Networking:  Add definitions and stuctures to support SIOCGIFCONF and SIOCGLIFCONF IOCTL commands.

Networking: sockgetname() files need to include udp/udp.h and tcp/tcp.h or otherwise NET_UDP_HAVE_STACK and NET_TCP_HAVE_STACK are undefined and the logic is never compiled.  Noted by Anthony Merlino

:Remove CONFIG_NET_6LOWPAN_FRAMELEN.  In this case where multiple radios are support3d, this may not be a constant.  6LoWPAN now always queries the driver to get the maximum frame length.

6LoWPAN: Remove the option to disable fragmentation support.  Two reasons:  (1) First fragementation is always required becaues IPv6 requires an MTU of 1280 bytes.  The is no application use case that can work without fragmentation support.  And (2) it greatly reduces the complexity of the code.

6LoWPAN/Radio:  Rename radio property sp_pktlen to sp_framelen.  Add 6LoWPAN utility to get the max frame length (not yet hooked in)

6LoWPAN: I believe, based on RFC review, that with the last multicast change, the NuttX 6LoWPAN is now compliant with RFC 6282.

networking:  IGMP: Remove special support for interrupt level processing (there is none) and fix some timer cancellation logic.  In many files, correct comments.  There is no interrupt level processing in the networking layer.