gethrxtime: don’t use nanouptime, microuptime

gethrxtime: don’t use nanouptime, microuptime

* lib/gethrxtime.c (gethrxtime): On platforms with
CLOCK_MONOTONIC, try it first.  If it fails, just fall back on
CLOCK_REALTIME as that’s what current_timespec would do anyway.
On platforms lacking CLOCK_MONOTONIC, just use current_timespec.
* m4/gethrxtime.m4 (gl_GETHRXTIME): Don’t check for microuptime or
nanouptime.  They aren’t exposed to user space by BSD kernels now,
and it’s not clear that they ever were.