Tor: Enable syscall sandbox

Tor: Enable syscall sandbox

This makes post-exploitation activities harder, in case the local Tor
instance has been compromised. It is worth noticing that Tor won't
respond to a "GETINFO address" command on the control port if sandboxed,
but our CGI does not make use of it, and neither is any legitimate
service on IPFire doing so.

Tested on a small middle relay running on an IPFire machine.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>