nixos: rewrite torsocks module

Rather than trying to override the 'torsocks' executable in $PATH, the
new module instead properly configures `/etc/tor/torsocks.conf` and puts
the normal `torsocks` executable in $PATH so it can work out of the box.

As a bonus, I think this module actually works now, because the torsocks
configuration has changed a lot from when this was written, it seems...

Signed-off-by: Austin Seipp <aseipp@pobox.com>