Redesign s_commod()

Redesign s_commod()

To implement the supply from self avoidance described in the previous
commit's message, s_commod() needs to be redesigned along the same
principles: take the sink as argument, update and put it.  Also take
an item limit argument, and return whether supply request was fully
met.

Update sct_supply(), shp_supply(), lnd_supply() and
lnd_could_be_supplied().  The former three become straighforward
wrappers.

supply_commod() and try_supply_commod() are now unused, remove them.