NETDB: Make sure that we have a valid DNS server address before attempting DNS queuries; Provide an optional default DNS server address in the configuration

This completes all interface level support for IPv6 DNS.  Still some missing logic at the lowest protocol levels

netdb: Modifications to DNS client logic and to dns_get/setserver() interfaces to support DNS clients at IPv6 addresses (still no support for resolution to IPv6 addresses).

- Rename include/nuttx/net/dnsclient.h to dns.h
- Move internal DNS prototypes from dns.h to libc/netdb/lib_dns.h
- Global standard DNS definitions from libc/netdb/dns_soccket.c to dns.h
- Eliminate dns_gethostip().  This is now an internal part of gethostbyname()
- Eliminate interface dns_whois().  Not needed in this new context.

DNS: Remove some unused, non-functional DNS functions; remove the annoying _sock at the end of some DNS functions names, it is no longer needed to distinguish the functions

samdl/sam_usb.h: fix USBDEV_CTRLB_SPDCONF defines Datasheet 03-2015 and 06-2015 is wrong. These defines are correct verified with analyser and also same values are used on Atmel xdk-asf-3.24.2 sources.  From Janne Rosberg.

Move the DNS server from apps/netutils/dnsclient to nuttx/libc/netdb.  Move netdb functions from libc/net to libc/netdb.  Fix up naming to reflect the repartitioning

PCA9555: Extend interface to include methods for reading more than one bit at a time. From Sébastien Lorquet.

Protected mode:  Redesign how the user space heap is accessed from the kernel code.  It used to call memory management functions in user space via function pointers in the userspace interface.  That is inefficient because the first thing that those memory management functions do is to trap back into the kernel to get the current PID.  Worse, that operation can be fatal is certain fragile situations such as when a task is exitting.

The solution is to remove all of the memory management function calls from the interface.  Instead, the interface exports the userspace heap structure and then kernel size implementations of those memory management functions will operate on the userspace heap structure.  This avoids the unnecessary system calls and, more importantly, failures do to freeing memory when a test exits.

More changes to LPC3450 pin configuration:  The hardware ETM pins will not toggle unless the SLEW FAST bit is set.  From  Alessandro Temil.

-headers in nuttx/ioexpander to define the "generic" ioexpander framework
-files in drivers/ioexpander for the implementation of the NXP PCA9555 framework
-bindings in drivers/Kconfig and drivers/Makefile

From Sebastien Lorquet