fix undead corpses being edible now

In 3.6.x, zombie corpses were always aged an extra 100:

NetHack/src/mon.c

Line 375 in 23d331a

 obj->age -= 100; /* this is an *OLD* corpse */
in 5.0.x, zombie corpses are always aged TAINT_AGE, which is only 50:

NetHack/src/mon.c

Line 648 in 97a6c13

 obj->age -= (TAINT_AGE + 1); /* this is an *OLD* corpse */

This is the result of commit 408321b.
The accompanying comment states that the purpose of that patch was meant to
just replace hard-coded numbers with symbolic values, but the commit set
two differing numeric values to the same symbol name, thus causing the
regression reported in:

https://github.com/NetHack/NetHack/issues/1664

Revert the values to match those of 3.6, and add the additional symbolic value.

Closes #1664

doc/fixes5-0-1.txt

@@ -87,6 +87,7 @@ upon return to a level, catch up on remaining nutrition value by the same
 	percentage as the catch up for the weight
 after polymorph or revival from corpse, monsters were not interacting with
 	the terrain until that monster's next move
+fix undead corpses (zombies, etc.) now being edible
 
 
 Platform- and/or Interface-Specific Fixes