client: Implement interactive mode.

client: Implement interactive mode.

This makes para_client enter an interactive session when started with no
command. Command line history and command completion are available in
interactive sessions.

This populates the previously empty files interactive.h and interactive.c which
contain the readline/interactive specific part. Everything in these files is
independent of para_client.  Conversely, client.c and client_common.c are
independent of readline.

The public API defined in interactive.h was designed to be reused from other
applications. In fact, a subsequent commit changes para_audioc to offer
interactive sessions as well.