Make DTMF attended transfer support feature-complete.

This greatly modifies the operation of DTMF attended transfers so that
the full range of options from features.conf applies.

In addition, a new option has been added that allows for a transferer
to switch between bridges during a transfer before completing the
transfer.

(closes issue ASTERISK-21543)
reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/2654



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395151 f38db490-d61c-443f-a65b-d21fe96a405b
Restore chan_dahdi native bridging and PRI tromboned call elimination.

Created a native_dahdi bridging technology for use with the new bridging
API.

The new bridging technology is part of the chan_dahdi channel driver
because it is very specific to that driver.  Rather than include the new
code directly into chan_dahdi.c the new bridge technology is in its own
file and linked into chan_dahdi.so.  A large part of this change is the
mechanical process of moving declarations around so chan_dahdi.c can be
split up into more files later.

* Changed the bridging core to pass NULL frames into the channel
technologies instead of discarding them.  The channel technologies may
need the proding to determine if their configuration is still valid.

(closes issue ASTERISK-21886)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2681/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395154 f38db490-d61c-443f-a65b-d21fe96a405b