Delete all socket files on servald start

If serval does not close properly, socket files are kept in
/data/data/org.servalproject/var/serval-node. Therefore, we need to clean up
when servald starts.
overlay_mdp_bind and overlay_mdp_getmyaddr should be visible in MDP API
Make overlay_mdp_client_socket_path local

Unlink mdp sockets by asking the OS what the bound name was.