monitor/misc.h has static add_completion_option().  It's useful
elsewhere in the monitor.  Since it's not monitor-specific, move it to
util/readline.c renamed to readline_add_completion_of(), and put it to
use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230124121946.1139465-7-armbru@redhat.com>