outbound.c: do not use OPTIONS keepalive by default but on UDP
outbound.c: do not use OPTIONS keepalive by default but on UDP

On TCP, use TCP-level keepalives.

darcs-hash:20080903174953-db55f-f01720d2915d2d83660a7957894123748f58f4dc.gz

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