retreat: Clean up interface between retreat_FOO(), retreat_FOO1()

retreat: Clean up interface between retreat_FOO(), retreat_FOO1()

Move clearing of retreat flags from retreat_ship(), retreat_land() to
retreat_ship1(), retreat_land1(), so it's where the retreat path is
shortened.

Move putship(), putland() from retreat_ship1(), retreat_land1() to
retreat_ship(), retreat_land(), so it's where the nxtitem() is, and
doesn't need a "if (!orig)" guard.  Requires making retreat_ship1()
and retreat_land() return non-zero when they modified their argument.

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