Undocumented misfeature: retreat and lretreat accept anything as
retreat path. The paths' actual consumers retreat_ship1() and
retread_land1() silently ignore invalid direction characters.
The retreat paths are in xdump, and invalid ones could conceivably
confuse smart clients.
Change the commands to reject invalid paths, and the consumers to oops
on invalid direction characters.
Note that invalid paths get rejected even when they're not actually
used because the conditions argument contains a "c" for "cancel".
Requiring the user give a new path so he can cancel the old one is
comically bad design.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>