chan_sip: Fix RTCP port for SRFLX ICE candidates

This corrects one-way audio between Asterisk and Chrome/jssip as a
result of Asterisk inserting the incorrect RTCP port into RTCP SRFLX
ICE candidates. This also exposes an ICE component enumeration to
extract further details from candidates.

(closes issue ASTERISK-21383)
Reported by: Shaun Clark
Review: https://reviewboard.asterisk.org/r/2967/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@402345 f38db490-d61c-443f-a65b-d21fe96a405b