load: Factor out plane_loadable(), land_loadable()

load: Factor out plane_loadable(), land_loadable()

load_plane_ship() and load_plane_land() duplicate code to check
whether a plane can be loaded, except for the phrasing of one message.
Factor out into plane_loadable().  tran_plane() has equivalent code,
but wants different messages, which makes de-duplication unattractive.

load_land_ship() and load_land_land() duplicate code to check whether
a land unit can be loaded.  Factor out into land_loadable().

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