retreat lretreat: Deprecate pseudo-condition 'c'

retreat lretreat: Deprecate pseudo-condition 'c'

It's redundant; retreat path 'h' cancels orders just fine.  Document
that instead.  'c' still works, and I don't plan to break it as long
as it doesn't get in the way, which seems unlikely.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>