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.

6LoWPAN: Correct a few addressing issues.  The apps/examples/udp test now appears to be fully functional.  Also reserve two bytes at the end of the frame for the FCS.

wireless/ieee802154: Minor naming change to align better with coding guidelines

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

Make naming of NTXDESC consistent.  I don't know where it should be MAC or IEEE, gut I changed the one that affects the fewest files.

pthread_mutex_unlock():  Missing check for the case where pthread_mutex_lock() is called when the mutex is not locked.  In that case, it would increment the underlying semaphore avove 1.  This is the fix for a problem noted by initialkjc@yahoo.com.

ieee802154 MAC: Free allocated TX descriptor if an error occurs after the descriptor has been allocated.