check_transfer_action

["Stanley Sutton" <sutton at t-surf dot com>]

In check_transfer_action, the unit2 is checked for acp_to_unload and
unit3 is always checked for acp_to_load, even when the amount transfered
is negative.  Transfer_supply, which reverses the units as far as
from/to goes when the amount is negative, does take into account which
is the from unit for unloading, and the to unit for loading.

Also, the acp_to_load and acp_to_unload seem to be strickly flags, as
they have no effect on the ACP to transfer material, which is hard coded
to 1.

An interesting side affect is that if one unit can never unload material
transfers material to a unit that can never load material, it will work,
as long as a negative amount of material is transferred, and the
max_unload and max_load tables for the respective units are non-zero.