It is expected to not output anything on stdout. Especially remote end
may be already terminated, so writing there would result in EPIPE.
FixesQubesOS/qubes-issues#1592
It would be called by qvm-sync-clock instead of 'date' directly. This
gives a lot of flexibility - VM can control whether it want to sync time
this way. For now slight corrections (+-2sec) are ignored to not cause
problems by frequent time changes. But it can be easily extended to
refuse time sync when some other mechanism is used.
