para_play, infrastructure.

para_play, infrastructure.

Now that the preparatory changes to the interactive subsystem
(single key mode, status bar, signal handling) and the writer nodes
(notifications) and receiver nodes (btr exec mechanism) are in place,
we can move on to the para_play executable.

This patch adds only the necessary changes to the build system and
provides a dummy implementation of para_play. The real implementation
will be provided as as a separate patch in the next commit.