services: tor: Make it easier to use UNIX sockets.

* doc/guix.texi (Networking Services): Document it, and mention that
tor-service is deprecated.
* gnu/services/networking.scm (<tor-configuration>) <socks-socket-type>:
New field.
(tor-configuration->torrc): When socks-socket-type is 'unix, set
SocksPort to UNIX domain socket /var/run/tor/socks-sock and set
UnixSocksGroupWritable to 1.
* gnu/tests/networking.scm (%tor-os/unix-socks-socket): Instead of using
a custom config file, just set socks-socket-type to 'unix.