Adds USB host support to stm32f411-disco board.

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

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.