Allow disabling the default ffwd/rewind keys in the ControlPlayback application.

This is done in a backward compat way.
If the "default" key for ffwd/rew is used for another option (such as stop), the "default" is removed.

(closes issue #11754)
Reported by: johan
Patches:
      app_controlplayback.c.option3.patch uploaded by johan (license 334)
Tested by: johan, qwell


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@101296 65c4cc65-6c06-0410-ace0-fbb531ad65f3