document the way sipdebug works, and implement it through

variables and not flags.

NOTE:
The old behaviour (preserved in this commit) is that if sipdebug
is set in the config file, it can only be disabled by reloading the
config.  I am not sure if this is accidental or voluntary, but it
is really unconvenient and I think it should be handled in the same
way as other options i.e. consider requests from the config file
or the cli (or the command line) to be fully equivalent and act on
the same status variable.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76330 65c4cc65-6c06-0410-ace0-fbb531ad65f3