unetmsg: only send publish notifications for remote publisher changes

unetmsg: only send publish notifications for remote publisher changes

handle_publish() notifies local subscribers about publisher state
changes. The publish/subscribe handler in network_socket_handle_request()
was calling it for both remote publish and subscribe changes, but
subscriber changes are not relevant to local subscribers.

Guard the handle_publish() calls with a msgtype == "publish" check,
matching the local client paths in unetmsgd-client.uc which already
have this guard.

Signed-off-by: Felix Fietkau <nbd@nbd.name>