Listen on both IPv6 and IPv4 when communication-method is inet_socket.

If communication-method is inet_socket, and IPv6 connectivity
is preferred between the client and server, programs like spd-say are
failing to connect because speech-dispatcher is only listening on IPv4.
It's easy to work around locally: just give a hardcoded IPv4 address.
But I shouldn't have to in 2020.
I am happy to send patches.

Inspect the speech-dispatcher communication path selected by `communication-method=inet_socket`, starting with the listener setup used when `spd-say` connects. Reproduce the failure with IPv6-preferred client/server connectivity and compare it with the hardcoded IPv4 workaround. Done means inet_socket accepts connections over both IPv6 and IPv4 without requiring a hardcoded IPv4 address.

Written by the indexing model from the issue text.