tlsdate: add optional --showtime support for "raw"

Add support for optional arguments to --showtime, "raw" and "human".

Human readable output is what --showtime and -V do already.  Passing
in --showtime=raw will tell tlsdate-helper to write the raw
unix time directly to stdout in host byte order.

The purpose of this change is to provide easier integration of
privilege isolation for time setting.  If tlsdate is called from
a wrapper, like tlsdated, it should be possible to pass the time back
to a privileged time-setter with the minimum of needed parsing.