su_addrinfo.c: use empty or NULL service for port 0
su_addrinfo.c: use empty or NULL service for port 0

Port 0 used as ephemeral port when bind()ing.

darcs-hash:20070228171850-55b16-3b068983d4adb2d67832f5e723143bdd472e17ed.gz

jart/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,jart/sofia-sip,jart/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip