More trouble in the supply code

[Lincoln Peters]

I have noticed two odd things happening in fantasy.g that are probably
due to malfunctioning supply and/or economy code and did not happen
prior to some of the recent updates (I'm not sure exactly which one): 

1. If a unit is building another unit with a consumption-per-build > 0
of any material (e.g. mana for a cloudkeep, metal for a tower), the unit
may end up with a negative supply of that material before it stops
building (e.g. one might find serfs with -2/6 metal).  And whenever a
unit's supply of any material drops below zero, the unit is completely
incapacitated until it can restore or receive supplies that pull it out
of "the red". 

2. Although the in-length for mana of an amulet of power is -1 (i.e.
never receives mana from anywhere or anything), I frequently see it
drawing mana from its transport (usually a wizard)!  It seems to me that
it should only be able to give mana to its transport, but never take
mana.