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 @
Sponsored by: NVIDIA Networking
Differential Revision: https://reviews.freebsd.org/D36858

(cherry picked from commit 0def80f1a5c8f7a02b92c823e5c71f9f746c3e6b)