Adds support for a core attended transfer function plus adds some hiding of masquerades.

Adds support for a core attended transfer function plus adds some hiding of masquerades.

The attended transfer API call can complete the attended transfer in a number of ways
depending on the current bridged states of the channels involved.

The hiding of masquerades is done in some bridging-related functions, such as the manager
Bridge action and the Bridge dialplan application. In addition, call pickup was edited
to "move" a channel rather than masquerade it.

Review: https://reviewboard.asterisk.org/r/2511

(closes issue ASTERISK-21334)
Reported by Matt Jordan

(closes issue Asterisk-21336)
Reported by Matt Jordan

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@389848 65c4cc65-6c06-0410-ace0-fbb531ad65f3