MFC r278817: touch: Fix some subtle bugs related to NULL times fallback:
* Do not subvert vfs.timestamp_precision by reading the time and passing
that to utimensat(). Instead, pass UTIME_NOW. A fallback to a NULL times
pointer is no longer used.
* Do not ignore -a/-m if the user has write access but does not own the
file. Leave timestamps unchanged using UTIME_OMIT and do not fall back to
a NULL times pointer (which would set both timestamps) if that fails.
git-svn-id: svn://svn.freebsd.org/base/stable/10@297783
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