comment and slightly restructure handle_request() in the part that handles

comment and slightly restructure handle_request() in the part that handles
responses, so that there is a common exit point.
Mark two places where probably we could return -1 instead of 0 to report
an error to the caller.
(change triggered by investigations on how the 'SIP_PKT_IGNORE' field was used).

nothing to backport from this commit

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76371 65c4cc65-6c06-0410-ace0-fbb531ad65f3