rpc: avoid unlinking sockets passed in from systemd

rpc: avoid unlinking sockets passed in from systemd

Currently the socket code will unlink any UNIX socket path which is
associated with a server socket. This is not fine grained enough, as we
need to avoid unlinking server sockets we were passed by systemd.

To deal with this we must explicitly track whether each socket needs to
be unlinked when closed, separately of the client vs server state.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>