MFC r314179 r314206 r314424

Add sem_clockwait_np()

This function allows the caller to specify the reference clock
and choose between absolute and relative mode. In relative mode,
the remaining time can be returned.

The API is similar to clock_nanosleep(3). Thanks to Ed Schouten
for that suggestion.

While I'm here, reduce the sleep time in the semaphore "child"
test to greatly reduce its runtime. Also add a reasonable timeout.

Relnotes: yes
Sponsored by: Dell EMC