Snapserver expects the arguments `--tcp.bind_to_address` and
`--http.bind_to_address` instead of the `--tcp.address` (and http
equivalent) versions.
This caused the process to listen on `0.0.0.0` (for TCP and HTTP
sockets) regardless of the configuration value. It also never listend on
the IPv6 address `::` as our module system made the user believe.
This commit fixes the above issue and ensures that (at least for the TCP
socket) that our default `::` does indeed allow connections via IPv6
(to localhost aka ::1).
* Add 'librespot' (new name for 'spotify'), 'alsa', 'tcp'.
* Add a warning about the spotify -> librespot rename.
* Fix the deprecated example `mode = "listen"` for type 'pipe'.
* Update the tests to include a straightforward 'tcp' test.
