Allow selection of network interface, improve registration and connection stability.

[mtwebster]

• Network interface can be either automatic or a specific interface can be selected.
• Use libnm for network handling rather than rolling our own.
• Additional port will be used for exchanging keys via an insecure grpc connection. This opens the possibility of eventually being able to add static addresses (not zeroconf-discovered).
• Primary connection was improved by: a) Waiting for confirmation of a duplex connection from the remote, rather than continuously pinging the remote until it succeeds. The check loop will be on the server side instead. b) Keepalive ping will be handled by grpc internally, which will signal if the connection drops.
• Institution of an 'api version' to be part of zeroconf service info, to signal which registration and connection method can be used between machines.
• The old connection/authentication methods remain in place for backward compatibility, so older and newer clients will still be able to connect. Refactoring was done to organize this a bit.