Save and restore AST_FLAG_BRIDGE_HANGUP_DONT on attended transfer.

Save and restore AST_FLAG_BRIDGE_HANGUP_DONT on attended transfer.

ast_bridge_call() clears AST_FLAG_BRIDGE_HANGUP_DONT. But during an attended
transfer, ast_bridge_call() is called for a second bridge on the same channel,
and it clears that flag, which still needs to get set for when the original
ast_bridge_call() gets control back and checks it.

Review: https://reviewboard.asterisk.org/r/741

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@277625 65c4cc65-6c06-0410-ace0-fbb531ad65f3