This only catches txtorcon not being installed (which should be fixed by
doing `pip install tahoe-lafs[tor]`). It doesn't notice that the Tor
daemon is not running (which we can't detect during startup, only
afterwards, when it's harder to notify the user), in which case Tor
connections (and all connections when "tcp = tor" is enabled) will just
fail silently.
Foolscap has limitations that prevent us from accepting anything but a
TCP endpoint, but that will change in the future, so make the tahoe.cfg
syntax accept an endpoint, but then reject non-TCP ones. See the ticket
for details: refs ticket:2813.
This depends upon the new `foolscap.connections.tor.socks_port(host,
port)` API in foolscap-0.12.2, so it bumps the dependency to that (the
previous commit depended upon 0.12.1, but I hadn't gotten around to
updating the dep before now).
Note that many of the Foolscap handler-creation functions are still
stubbed out, so Tahoe won't be able to honor the full range of config
syntax until foolscap support is complete.
