add IPtables chain for outgoing Tor traffic

add IPtables chain for outgoing Tor traffic

If Tor is operating in relay mode, it has to open a lot of outgoing
TCP connections. These should be separated from any other outgoing
connections, as allowing _all_ outgoing traffic will be unwanted and
risky in most cases.

Thereof, Tor will be running as a dedicated user (see second patch),
allowing usage of user-based IPtables rulesets.

Partially fixes #11779.

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