Updates command registration to provide top-level handlers for all
commands, rather than falling back onto the 'unknown' command. Instead,
that same handler is registered for placeholders, providing the same
functionality under the root verb command name instead.  This permits
users to implement their own 'unknown' function, and it resolves some
mind-bending breakage related to function object lookup while recursing.

Changes 'ocd_bounce' to call 'ocd_command' and 'ocd_help' from the
wrapper directly, rather than bouncing through their wrappers. This
prevents endless recursion caused by the above changes, whereby the
'command' wrapper's type check would blow the stack to hell and gone.