Ignore-this: 4e7fdc56065dba617352443a9310bb28

Use timer D (instead of timer C) to timeout CANCELed INVITE transactions.

Also, generate 408 Request Timeout to all forks that have not received a
final response.

darcs-hash:20090303164700-db55f-c6f7c892e7226f0daf8f8c7e86482ffd2be457d1.gz