Respond to requests only from the configured SIP proxy

An incoming SIP request will be silently dropped if the following conditions are all met:
- The request is about to create a new session
- The source IP address of the request is not the address of the configured proxy

Note
- The source port of the request is not checked since we have not found the good way to
  retrieve the port that is used for transmission
- If the configured proxy is a domain name other than IP address, the resolved IP address
  might be different from the one which is being used in existing sessions