Add a --config-test option

Add a --config-test option

It lets Privoxy exit after checking whether or not the
configuration seems valid. The limitations noted in
TODO #22 and #23 still apply.

Logging the problem to the logfile if one is configured
and --no-daemon isn't used could be considered non-intuitive
and might also clobber messages emitted by an already running
instance.

Based on a patch by Ramkumar Chinchani.