hero health manipulation

I've been investigating issue #1252 (while the fuzzer was running,
sanity_check complained that hero's current health was greater
than maximum health) off and on for three months and haven't found
the cause.

I've checked all the places that lower maximum HP that I've managed
to find, but not spent much time looking for places that raise
current HP.

These changes might provide some more information.  They don't rely
on sanity_check being enabled.

Issue #1252 is still open.

src/hack.c

@@ -4039,10 +4039,15 @@ maybe_wail(void)
 int
 saving_grace(int dmg)
 {
-    if (!u.usaving_grace && (u.uhp <= dmg)
-        && (u.uhp * 100 / u.uhpmax) > 90) {
+    if (dmg < 0) {
+        impossible("saving_grace check for negative damage? (%d)", dmg);
+        return 0;
+    }
+
+    if (!u.usaving_grace && dmg >= u.uhp && (u.uhp * 100 / u.uhpmax) > 90) {
+        /* note: this could reduce dmg to 0 if u.uhpmax==1 */
         dmg = u.uhp - 1;
-        u.usaving_grace = TRUE; /* used up */
+        u.usaving_grace = 1; /* used up */
         end_running(TRUE);
         if (u.usleep)
             unmul("Suddenly you wake up!");