res_agi: Prevent an AGI from eating frames it should not. (Re-do)

res_agi: Prevent an AGI from eating frames it should not. (Re-do)

A dialplan intercept routine is equivalent to an interrupt routine.  As
such, the routine must be done quickly and you do not have access to the
media stream.  These restrictions are necessary because the media stream
is the responsibility of some other code and interfering with or delaying
that processing is bad.  A possible future dialplan processing
architecture change may allow the interception routine to run in a
different thread from the main thread handling the media and remove the
execution time restriction.

* Made res_agi.c:run_agi() running an AGI in an interception routine run
in DeadAGI mode.  No touchy channel frames.

ASTERISK-25951

ASTERISK-26343

ASTERISK-26716

Change-Id: I638f147ca7a7f2590d7194a8ef4090eb191e4e43