Modify the SDP parsing code to parse session and media level items separately.

With the new code, media level proprieties should no longer be confused with session level proprieties. This change also reorganizes some of the SDP parsing code which should make it easier to manage in the future.

(closes issue #14994)
Reported by: frawd
Tested by: frawd, mnicholson, file

Review: https://reviewboard.asterisk.org/r/414/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@227759 65c4cc65-6c06-0410-ace0-fbb531ad65f3