- Doxygen and comments updates

- Moving structure declarations to top of file with the rest
- Adding some forward declarations for RTP interface functions

(All these changes to position in file are in preparation for splitting chan_sip up
into several files at some point in the future)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8932 65c4cc65-6c06-0410-ace0-fbb531ad65f3