Allow chan_sip to decline unwanted media streams

This change replaces the static array of four representable media
streams with an AST_LIST so that chan_sip can keep track of offered
media streams.  This allows chan_sip to deal with offers containing
multiple same-type streams and many other situations without rejecting
the SDP offer in its entirety, yet still generating a valid response.
This also covers cases where Asterisk can not comprehend the offer if
it is in the correct format.

Previously, chan_sip would reject SDP offers or entirely ignore
individual stream offers in an effort to be more compatible which
would often result in invalid SDP responses.

Review: https://reviewboard.asterisk.org/r/1988/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369028 65c4cc65-6c06-0410-ace0-fbb531ad65f3