- Adding doxygen comments

- Adding doxygen comments
- Changing default values set in reload_config to DEFAULT_ #defines to make it more clear what defaults are
- Cleaning up global_ and default_ variable naming.
- Moving variable and #defines together in the source, adding comments to explain sections

Global_ is used for channel settings that does not apply to peers or users as defaults for their settings
default_ is used both as a channel setting for unknown callers, as well as defaults for peers and users

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8514 65c4cc65-6c06-0410-ace0-fbb531ad65f3