Fixup hold/unhold with attended and blind transfers.

* DTMF attended and blind transfers have hold/unhold behavior restored.

* External attended and blind transfers unhold the transfered party when
the transfer is initiated.

* Made prohibit blind transferring a bridge marked as masquerade only.
(ConfBridge bridges)

* Made running an application or playing a file inside a bridge post the
hold/unhold messages if MOH is requested.

Review: https://reviewboard.asterisk.org/r/2574/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390289 65c4cc65-6c06-0410-ace0-fbb531ad65f3