annoying shop bug

annoying shop bug
     The shop billing code assumes that food marked partly eaten
is worthless, but the eating code was changed to make sure that any
eating attempt will never leave food marked as not partly eaten.
The end result is that non-corpse food which is consumed in a
single byte can be eaten off shop floors for free, and if eaten
from inventory--so already on shop bill--the "Ix" display of used
up goods lists the item as partly eaten (with right price though).

     This fix makes single-byte food be handled the same as other
food:  the first byte taken causes the food to be auto-purchased
immediately instead of waiting until it's used up to add it to the
shopping bill.