bgpd: start listener on first instance

bgpd: start listener on first instance

Start BGP listener only after first instance is started.  This helps the
security if BGP is not used but daemon is started.  It also addresses some
issues like MD5 not working on listener unless IPV6 configured (because
listener was not in list); as well as compiler warnings.

* bgp_network.c: (bgp_listener) listen socket creation consolidated here
  (bgp_socket) Use bgp_listener
* bgpd.c: (bgp_get) call bgp_socket on creation of first struct bgp.
  (bgp_init) remove bgp_socket call.
* memtypes.c: Add MTYPE_BGP_LISTENER