TUN driver: Implement TAP (OSI layer 2) mode.

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

Enable by setting the IFF_TAP flag instead of the IFF_TUN flag in ifr_flags.

libc/netdb: in dns_query_callback, ret != -EADDRNOTAVAIL condition consumes error returns including EAGAIN in this case, dns query retransmission doesn't work

Add logic to disable cancellation points within the OS.  This is useful when an internal OS function that is NOT a cancellation point calls an OS function which is a cancellation point.  In that case, irrecoverable states may occur if the cancellation is within the OS.

STM32F7: serial: disallow broken configuration combination of CONFIG_STM32F7_FLOWCONTROL_BROKEN=y and CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS not set.

pthreads:  Backed most of last pthread changes.  Found the 'real' root poblem.  A one like error in pthread_mutex.c.

examples/ostest:  pthread rwlock cleanup handlers must call pthread_consistent, not pthread_mutex_unlock() on cancellation if robust mutexes are enabled.

pthreads: Re-order some operations so that mutexes are placed in the inconsistent state BEFORE the clean-up callbacks are called.

Photon:  Add logic to automatically mount the procfs file system on startup.  Fix some LED-related configuration conflicts.

stm32 COMP:  Logic in stm32_comp.h must be configured on CONFIG_STM32_COMP or otherwise it causes an error via #error on every platform without COMP support.

net procfs: Some long lines were being generated that cause buffer-related problems and corrupted output

Add support for NuttX controlled LEDS and for board_initialize.  Separate initialize logic to stm32_bringup.c so that in initialization can occur either through board_initialize() or through board_app_initialize().  Same as with most other newer board configurations.

Fix photon/nsh/defconfig, was turning on non-existent LED support when the configuration was refreshed.

Buttons: Change return value of board_buttons() and the type of btn_buttonset_t to uint32_t so that more than 8 buttons can be supported.

6loWPAN: Fix breakage in IPv6 dispatch caused by fixes to HC1 dispatch; Move some standard definitions from internal header file to include/nuttx/net/sixlowpan.h.  Update a README.

pthread.h:  Remove duplicate, possible erroneous definitino of PTHREAD_MUTEX_INITIALIZER that crept in with some recent changes.

Restore TCP_HDRLEN to MSS calculation.  Also add to UDP MSS calculation where it never appearred.  Add some missing MSS and RDVWNDO definitinos for 6loWOPAN.