Fix return value of s_commod() when it can't draw enough
s_commod() could incorrectly claim success when the sink ended up with
at least as many supplies than were missing initially. This caused a
number of problems:
* shp_torp() let a ship with two shells fire a torpedo, resulting in
-1 shells, which then made item_prewrite() oops. Affected missions
and return fire, but not the torpedo command.
* shp_missile_defense() let a ship with one shell use missile defense,
resulting in -1 shells, and the same item_prewrite() oops.
* Land units were considered in supply even when they had not quite
enough supplies. Such land units could defend without penalty,
attack and react. Commands load and lload weren't affected, because
they use lnd_in_supply(), which doesn't use s_commod().
Broken in
98f24d5c, v4.3.20.