clock_gettime: Add Linux aliases for CLOCK_*

Linux standardized what we call CLOCK_{REALTIME,MONOTONIC}_FAST as
CLOCK_{REALTIME,MONOTONIC}_COARSE. In addition, Linux spells
CLOCK_UPTIME as CLOCK_BOOTTIME.

Add aliases to time.h and document these new aliases in
clock_gettime(2).

Reviewed by: vangyzen, kib (prior), dchagin (prior)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30988

(cherry picked from commit 155f15118a77d2aeab7b177ada78c848778d7d80)