Rename the SendFAX/ReceiveFAX 'force audio' option.

The recently added option to disable usage of T.38 for a single
session should have been named 'F' for 'force audio', since that
is really what the user is asking to happen (and it's a positive
option instead of a negative option that way).



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@304422 65c4cc65-6c06-0410-ace0-fbb531ad65f3