Remove requirement of object probs adding to 1000

When discussing the recent commit that removed makedefs -o from the
build process, nhmall pointed out that a sanity check ensuring all
objects within one class add up to 1000 probability had been removed as
well. This requirement was a perennial thorn in the side for anyone
doing anything that touches object probabilities, because allocating
probability to something meant deciding what to take it away from,
without a good way to evenly distribute that across all the other
members of the object class.

I had gotten around this in xNetHack by removing the sanity check and
making mkobj() total up the probability within an object class and then
using that instead of 1000. This commit takes a similar approach, but
instead of inefficiently recalculating the sum every time mkobj() is
called, it instead computes it at the start of the game or when
restoring the save file and stores it in a global variable.

This fixes a slight bias problem with rings - they are all supposed to
be of equal probability, but there are 28 of them and 1000 is not evenly
divisible by that, so the old formula made the later rings slightly more
likely. Now instead of a 35/1000 or 36/1000 chance, they are all
uniformly 1/28. (Internally they have a oc_prob of 1 now, not 0).

Gems are also weird, because their oc_prob values change every level.
This ought to have still worked without a change, because the arcane
formula for assigning the probabilities would still end up with them
adding to 1000. But I added in code to reset the total gem probability
anyway; this may help make the formula less arcane in the future.

There is still a sanity check against object classes having a nonzero
number of objects but zero total probability, in which case an
impossible will be thrown and every member of the class will be given
equal probability. I also downgraded the "probtype error" panic in
mkobj() to an impossible because it has a reasonable failure case -
return the first item in that class.

include/objects.h

@@ -651,7 +651,7 @@ BOOTS("levitation boots", "snow boots",
     OBJECT(OBJ(name, stone),                                          \
            BITS(0, 0, spec, 0, mgc, spec, 0, 0, 0,                    \
                 HARDGEM(mohs), 0, P_NONE, metal),                     \
-           power, RING_CLASS, 0, 0, 3, cost, 0, 0, 0, 0, 15, color,sn)
+           power, RING_CLASS, 1, 0, 3, cost, 0, 0, 0, 0, 15, color,sn)
 RING("adornment", "wooden",
      ADORNED,                  100, 1, 1, 2, WOOD, HI_WOOD, RIN_ADORNMENT),
 RING("gain strength", "granite",