nua_subnotref.c: do not terminate SUBSCRIBE usage if Expires: 0 is used
nua_subnotref.c: do not terminate SUBSCRIBE usage if Expires: 0 is used

Terminate subscription only when NOTIFY is received or timeout occurs.

darcs-hash:20080808115501-db55f-56062f3b43d19c370b595b063dceb16b76ef65e3.gz

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