lib/util: Standardize use of st_[acm]time ns

lib/util: Standardize use of st_[acm]time ns

Commit 810397f89a10, and possibly others, broke the build for macOS and
other environments which don't have st_[acm]tim fields on 'struct stat'.

Multiple places in the codebase used the config.h values to determine
how to access the nanosecond or microsecond values of the stat
timestamps, so rather than add more, centralize them all into
lib/util/time.c.

Also allow pvfs_fileinfo.c to read nanosecond-granularity timestamps on
platforms where it didn't before, since its #if branches were not
complete.

Signed-off-by: Matthew DeVore <matvore@google.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Sat Aug 15 08:51:09 UTC 2020 on sn-devel-184