ieee80215: starts adding support for beacon-enabled networking

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

6LoWPAN TCP: Fix more ACK handling, fix some callback flag settings.  Now the TCP test appears fully functional on 6LoWPAN.

6LoWPAN: Modify the the way TCP headers are transferred -- yet again.  TCP packet reassembly now seems to work OK, but no there now does not seem to be a listener on the port.

6LoWPAN: Various fixes for a clean build if either TCP or UDP are disabled.  Given the current state of TCP, it is recommended that you disable TCP.

6LoWPAN TCP: Major re-architecting of TCP logic to properly handle TCP stuf like ACKs and TPC windowing which were not properly covered in the initial design.  Still does not work; hangs waiting of ACKs.

6LoWPAN: TCP logic was not obeying MTU packet size limitations.  Other TCP-specific issues also fixed.  There remains a major outstanding issue with ACK handling.

clicker2-stm32/mrg24j40-6lowpan: Add apps/examples/nettest.  Refresh all configurations that enable the nettest.

ieee802154: Minor renaming to align better with coding style standard

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

drivers/net/tun.c: Use critical section instead of semaphore in tun_ifdown()

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

netdb:  Fixed buffer size used for sending DNS queries should depend on the configured DNS name size.

6LoWPAN:  fix yet another addressing problem.  That does it.  Several recent, proper bugfixes unmasked a few errors in the 6LoWPAN logic.  But I think full functionality is restored.

6LoWPAN:  Loopback driver needs to initialize the MAC meta data; Address decompression logic must have the MAC address to handle the most common compression cases.

Do not search net device when all-zeros address is used.

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

Fix problems in last commit noted by Jeongchan Kim:  last change returned -EINVAL vs EINVAL; Treat the case where the mutex is already unlocked just like the case where the mutex is held by another thread -- return EPERM.