add new channel option (via ast_channel_setoption()) to let channel drivers...

add new channel option (via ast_channel_setoption()) to let channel drivers adjust txgain/rxgain if they are able (only Zap channels at this time)
modify app_chanspy to use new gain option
reformat app_chanspy to match coding guidelines
add user-controlled volume adjustment to app_meetme (issue #4170, heavily modified to actually work on Zap channels)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6519 65c4cc65-6c06-0410-ace0-fbb531ad65f3