We cannot (easily) use clock_gettime(CLOCK_MONOTONIC) directly as

We cannot (easily) use clock_gettime(CLOCK_MONOTONIC) directly as
it may be present but not implemented.  Add sudo_gettime_real() and
sudo_gettime_mono() functions to get the real and monotonic times
respectively.  Now sudo_gettime_mono() checks the value of
sysconf(_SC_MONOTONIC_CLOCK) before calling clock_gettime(CLOCK_MONOTONIC)
and falls back on sudo_gettime_real() as needed.  The Mach version
of sudo_gettime_mono() uses mach_absolute_time().

This should fix problems with timestamp files on systems where
the CLOCK_MONOTONIC is defined but not actually implemented.