Added "encryption" option to "Audio" section of config read by sip_audio_session.py, can be one of "disabled", "optional" (default) or "mandatory", sets SRTP options

darcs-hash:20081202163328-58103-9c602ed438e27f15178c31904c67c87ee1d4b55f
Added extra port checking to PJSIPEndpoint init

darcs-hash:20081202165042-58103-073bb9344d3fd284c4f210acee7c017e48fa2518