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.