Move some global 'flags' to individual variables.

Move some global 'flags' to individual variables.

Start putting these variables in a single struct (called 'sip_cfg' for the time
being, but it could as well be 'global' or some other name) so it
is easy, when reading the code, to figure out what they are for.

The downside of using struct fields instead of individual global
variables is that the compiler cannot tell if there are unused fields.
But the advantage of not cluttering the namespace and manilpulating
all these variables at once certainly overcome the disadvantagess.

Nothing to backport, again.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@77711 65c4cc65-6c06-0410-ace0-fbb531ad65f3