Update ZapShowChannels so that you can specify one channel.

Action ZapShowChannels
        Header changes
        - Channel:      -> ZapChannel
        For active channels, the Channel: and Uniqueid: headers are added
        You can now add a "ZapChannel: " argument to zapshowchannels actions
        to only get information about one channel.

From the moremanager branch


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@91386 65c4cc65-6c06-0410-ace0-fbb531ad65f3