wdogs: Fix counting of free, pre-allocated wdog timers.  The could could get decremented below zero in some situations

recvfrom(): Correct wait for new data when NET_UDP_READAHEAD is enabled.  Fix size accounting when recvfrom_udpreadahead() sets state.rf_recvlen == -1.  I have not checked if data are accumulated to the right position in the buffer however.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>

sem_tickwait():  Add a new, non-standard function to perform timed semaphore waits.  It is functionally equialent to the standard sem_timedwait(), but more efficient for use in higher performance device drivers.  Requested by Max Neklyudov

sem_tickwait():  Add a new, non-standard function to perform timed semaphore waits.  It is functionally equialent to the standard sem_timedwait(), but more efficient for use in higher performance device drivers.  Requested by Max Nekyudov

16550 UART Driver: Add a configuration option to indicate the the THR empty bit is inverted.  This is the the case for the moxART SoC.  Based comments from Anton D. Kachalov

sched/: Separate the round-robin logic into a separate file so that it is symmetric with the sporadic stuff.  Integrate the sporadic scheduler into the time tick interrupt handling and into the tickless operation.

include/, sched/, and libc/: Add support for sporadic scheduling parameters in struct sched_param, posix_spawnattr_t, and pthread_attr_t. Update all user interfaces to pass sporadic scheduling parameters.  Feature is dependent on EXPERIMENTAL and no changes have yet been made to core scheduling logic.

pthread_create: Fix an (unlikely) error in fallback value in the event of a failure (which should never occur)

TCB: Increase the size of the scheduling policy field from 1 to 2 bits to allow additional, planned scheduling policies