Tor: allow multiple countries to be selected for Exit relays

Tor: allow multiple countries to be selected for Exit relays

This extends the functionality of the Tor CGI in order to be able to
select multiple countries for possible Exit relays, which is - in terms
of anonymity - less worse than limiting all Tor circuits to a single
country.

For example, a user might want to avoid Exit relays in more than one
country, and permit Tor to use Exit relays elesewhere, and vice versa.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>