Add UnixSocketsGroupWritable config flag

When running a system-wide instance of Tor on Unix-like systems, having
a ControlSocket is a quite handy mechanism to access Tor control
channel.  But it would be easier if access to the Unix domain socket can
be granted by making control users members of the group running the Tor
process.

This change introduces a UnixSocketsGroupWritable option, which will
create Unix domain sockets (and thus ControlSocket) 'g+rw'. This allows
ControlSocket to offer same access control measures than
ControlPort+CookieAuthFileGroupReadable.

See <http://bugs.debian.org/552556> for more details.