Add the ability to retrieve the source port of a SIP call.

This adds the ability to call CHANNEL(recvport) on chan_sip
channels to see the port on which an INVITE was received.

ASTERISK-24040 #close
Reported by dtryba
Patches:
	dialplan_functions.patch uploaded by dtryba (License #6628)

Review: https://reviewboard.asterisk.org/r/3781



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419970 65c4cc65-6c06-0410-ace0-fbb531ad65f3