miniupnpc-libevent: split upnpc_init() / upnpc_start()
miniupnpc-libevent.h: remove unused definitions
miniupnpc-libevent.c: free event_cif_url/event_conn_url

see 2455cc85b65106fdd1e2335786d5a357b6b561e8
miniupnpc-libevent: upnpc_set_local_address()
miniupnpc-libevent: include port in Host HTTP header if != 80
miniupnpc-libevent: subscribe and receive UPNP events !

enable with -DENABLE_UPNP_EVENTS
libevent need to support SUBSCRIBE and NOTIFY requests
see https://github.com/miniupnp/libevent/tree/upnp_ext
miniupnpc-libevent: "fix" debug_printf() if DEBUG not defined.