this adds the protocol registry. we also extend the systemd unit setup
to allow adding new protocols without much fussing, and we move deamon
instances into protocol-dependent scopes. this will make monitoring or
resource limiting of individual protocol daemon instances much easier.
Change-Id: Ic9da08eb074868b6e57385075c5502b081e8461f
this will let us configure more than one socket to connect/bind to,
which in turn lets us use posix acls on sockets for access control.
we will also need something like this for the final rpc transition.
Change-Id: I9c39f14906e9bf809055ab5c94bf687745b4f69e