Improve timeout precision of pthread_cond_timedwait().

Improve timeout precision of pthread_cond_timedwait().

This code was not touched when all other user-space sleep functions were
switched to sbintime_t and decoupled from hardclock.  When it is possible,
convert supplied times into sbinuptime to supply directly to msleep_sbt()
with C_ABSOLUTE.  This provides the timeout resolution of few microseconds
instead of 2 milliseconds, plus avoids few clock reads and conversions.

Reviewed by:	vangyzen
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D34163