Tor: allow enforcing distinct Guard relays or countries

Tor: allow enforcing distinct Guard relays or countries

In order to make deanonymisation harder, especially high-risk Tor users
might want to use certain Guard relays only (for example operated by
people they trust), enforce Tor to use Guard relays in certain countries
only (for example countries with very strict data protection laws or
poor diplomatic relations), or avoid Guard relays in certain countries
entirely.

Since Tor sticks to sampled Guards for a long time (usually within the
range of months), restricting those is believed to cause less harm to a
users' anonymity than restricting Exit relays, since their diversity of
a generic Tor user is significantly higher.

This patch extends the Tor CGI for restricting Guard nodes to certain
countries or relays matching certain fingerprints.

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