time(3): Align fast clock times to avoid firing multiple timers.

In non-periodic mode absolute timers fire at exactly the time given.
When specifying a fast clock, align the firing time so that less
timer interrupt events are needed.

Reviewed by: rrs @
Differential Revision: https://reviews.freebsd.org/D36858
MFC after: 1 week
Sponsored by: NVIDIA Networking