Change s_commod() not to use the supply sink as source. As explained
in the message of the commit before the previous one, using the sink
as source makes it impossible for callers to safely keep a copy of the
sink across a supply call. All current users do that. Some were safe
anyway, some were not:
* fort_fire() was safe, because a fort draws shells only when it has
none.
* shp_fire() was unsafe for ships with capability supply and ammo use
greater than 1. No such ship exists in the stock game.
* shp_dchrg() was unsafe for ships with both capabilities dchrg and
supply. Same for shp_torp() and capability torp, and
shp_missile_defense() and capability anti-missile. No such ship
exists in the stock game.
* lnd_fire(), supp() and get_dlist() were safe, because they draw
shells only when they have less than their ammo need, and then they
don't supply any.
* mission_pln_equip() was unsafe when equipping planes with shells in
supply sources.
* landmine() was unsafe for land units with both capability engineer
and supply. No such land units exist in the stock game.
* load() and lload() were unsafe for loadable supply units, but the
supply use there was disabled in commit
65410d16 because of another
bug.
* ask_olist() and att_reacting_units() were safe, because
lnd_can_attack() excludes supply units.
In the stock game, planes flying interception or support missions,
abms intercepting ballistic missiles, launch of missiles or anti-sats
could conjure up shells, triggering a seqno mismatch oops.
In games with unusual customizations, this could also happen with
supply ships firing guns or torpedoes, dropping depth charges, or
shooting down marine missiles, and in the lmine command.