- constification of some functions (args and return values):

- constification of some functions (args and return values):
        transmit_response_reliable()
        hangup_cause2sip()
- remove useless braces;
- add comments on some slightly cryptic code segments
- mark XXX possible critical pieces of code.
- remove an unneeded string termination after ast_copy_string
- mark usage of some rarely used functions;
- use strsep() instead of emulating it inline;
- replace magic constants with sizeof(array)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@18792 65c4cc65-6c06-0410-ace0-fbb531ad65f3