net/tcp:  In tcp_input(), add the logic to detect, decode, and respond to TCP Keep-Alive packets.  In tcp_timer(), fix some problems with the way that TCP Keep-Alive probes were being formatted.

    net/tcp:  Add logic to send probes when SO_KEEPALIVE is enabled.
    net/tcp:  TCP socket should not have to be connected to configure KeepAlive.
    net/: Add a separate configuration to enable/disable KEEPALIVE socket options.
    net/tcp: Arguments to TCP keep-alive timing functions probably should be struct timeval as are the times for other time-related socket options.
    net/tcp:  Fix a backward conditional
    net/tcp:  Add some more checks and debug output to TCP-protocol socket options.
    net/tcp:  Cosmetic changes to some alignment.
    net/:  Adds socket options needed to manage TCP-keepalive and TCP state machine logic to detect if that the remote peer is alive.  Still missing the timer poll logic to send the keep-alive probes and the state machine logic to respond to probes.

net/tcp: tcp_send_[un]buffered.c:  Check routing table in psock_send_addrchck().  Previously only ARP table was considered when determining if the data will actually be sent.

Added ECANCELED condition to DEBUGASSERT-s checking sem_wait result

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Fixed misprint in ipv4_forward.c

Approved-by: Gregory Nutt <gnutt@nuttx.org>

net/tcp:  Fixed bad return value handling in psock_tcp_send().  send() expects psock_tcp_send() to return a negated errno value, not -1 with the errno set (GN: I added same change for tcp_send_buffered.c which has the same issue as tcp_send_unbuffered.c)

    binfmt/, configs/, grahics/, libc/, mm/, net/, sched/:  OS references to the errno variable should always use the set_errno(), get_errno() macros
    arch/arm/src/stm32 and stm32f7:  Architecture-specific code is not permitted to modify the errno variable.  drivers/ and libc/:  OS references to the errno variable should always use the set_errno(), get_errno() macros

net/udp:  Fix memory leak with UDP + write buffer is closed.  Also update TODO and comments and refresh a configuration.

net/udp:  Resolves final design issues with UDP write buffering.  100% code complete but also 100% untested.

Squashed commit of the following:

    net/udp:  Address most of the issues with UDP write buffering.  There is a remaining issue with handling one network going down in a multi-network environment.  None of this has been test but it is certainly ready for test.  Hence, the feature is marked EXPERIMENTAL.
    net/udp:  Some baby steps toward a corrected write buffering design.
    net/udp:  Remove pesky write buffer macros.
    Eliminate trailing space at the end of lines.
    net/udp:  A little more UDP write buffering logic.  Still at least on big gaping hole in the design.
    net/udp:  Undefined CONFIG_NET_SENDTO_TIMEOUT.
    net/udp:  Crude, naive port of the TCP write buffering logic into UDP.  This commit is certainly non-functional and is simply a starting point for the implementatin of UDP write buffering.
    net/udp:  Rename udp/udp_psock_sendto.c udp/udp_psock_sendto_unbuffered.c.

net/udp:  Remove some conditional logic that was true if there is only a single network device, but not true in the multi-device context.

net/tcp:  Write buffering logic should not wait for a free buffer if the socket was opened non-blocking.  Also, rename the TCP write buffering macros from WRB_* to TCPWB_* to make room in the namespace for write buffering with other protocols.

net/udp:  Fix memory leak with UDP + write buffer is closed.  Also update TODO and comments and refresh a configuration.

net/udp:  Resolves final design issues with UDP write buffering.  100% code complete but also 100% untested.

Squashed commit of the following:

    net/udp:  Address most of the issues with UDP write buffering.  There is a remaining issue with handling one network going down in a multi-network environment.  None of this has been test but it is certainly ready for test.  Hence, the feature is marked EXPERIMENTAL.
    net/udp:  Some baby steps toward a corrected write buffering design.
    net/udp:  Remove pesky write buffer macros.
    Eliminate trailing space at the end of lines.
    net/udp:  A little more UDP write buffering logic.  Still at least on big gaping hole in the design.
    net/udp:  Undefined CONFIG_NET_SENDTO_TIMEOUT.
    net/udp:  Crude, naive port of the TCP write buffering logic into UDP.  This commit is certainly non-functional and is simply a starting point for the implementatin of UDP write buffering.
    net/udp:  Rename udp/udp_psock_sendto.c udp/udp_psock_sendto_unbuffered.c.

net/udp:  Remove some conditional logic that was true if there is only a single network device, but not true in the multi-device context.

net/tcp:  Write buffering logic should not wait for a free buffer if the socket was opened non-blocking.  Also, rename the TCP write buffering macros from WRB_* to TCPWB_* to make room in the namespace for write buffering with other protocols.

net/route:  Fix a couple of compile-related issues that have crept in sense the last time the file-based routing table was used.

    fs: Add truncate() support for userfs
    fs/unionfs:  Add truncate() support to the unionfs
    fs/tmpfs:  Add ftruncate() support to tmpfs
    syscall/: Add system call support for ftruncate()
    net/route:  Adding ftruncate() support eliminates an issue in file-based routing table management.
    fs:  Add basic framework to support truncate() and ftruncate().  The infrastructure is complete.  Now, however, the actual implementation of ftruncate() will have to be done for each file system.

net/arp:  Fix IGMP multicast address computation on Nuttx network stack.  This change fixes the IGMP address computation to allow multicast UDP messages. The destination address was created with the incorrect bytes of the given IPv4 address.

arch/arm/src/lpc54xx:  Correct handling of the Ethernet RBU error.  With this fix, Ethernet now appears to be fully functional although still undertested.

net:  Fix an error introduced when ICMP and ICMP6 socket support was added in NuttX-7.3.  A gratuitous ARP (or solitication) was being sent after receive of the ECHO replay (advertisement).