MFC r278817: touch: Fix some subtle bugs related to NULL times fallback:

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 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f