Add a flag to disable the Background behavior, for AGI users.

Add a flag to disable the Background behavior, for AGI users.
This is in a section of code that relates to two other issues, namely
issue #14011 and issue #14940), one of which was the behavior of
Background when called with a context argument that matched the current
context.  This fix broke FreePBX, however, in a post-Dial situation.
Needless to say, this is an extremely difficult collision of several
different issues.  While the use of an exception flag is ugly, fixing all
of the issues linked is rather difficult (although if someone would like
to propose a better solution, we're happy to entertain that suggestion).
(closes issue #16434)
 Reported by: rickead2000
 Patches:
       20091217__issue16434.diff.txt uploaded by tilghman (license 14)
       20091222__issue16434__1.6.1.diff.txt uploaded by tilghman (license 14)
 Tested by: rickead2000

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@237405 65c4cc65-6c06-0410-ace0-fbb531ad65f3