fix issue #1248 - drowning on Plane of Water

Issue reported by Umbire:  if hero dies by drowning on the Plane
of Water, cause of death was reported as "drowning in a limitless
water".

Reported for livelogging but applied to tombstone and logfile too.
Omit the article "a" in this situation.

For 3.6.7, it would have started as "drowning in a water" and been
updated on the fly to be "drowning in deep water".  3.7 changed
terrain type WATER to be "wall of water", where "a" is expected,
and also added "limitless water" for Plane of Water, but it was
neglecting to include a similar fixup for the latter.  The "deep
water" fixup is still present but doesn't get triggered anymore.

Fixes #1248

doc/fixes3-7-0.txt

@@ -1949,6 +1949,7 @@ when chain lightning hit a sleeping monster, the victim didn't wake up
 on medusa-4, Medusa and the downstairs were located in different chambers
 on medusa-3 and medusa-4, Perseus's statue was located in a different chamber
 	from Medusa
+omit "a" from "drowned in a limitless water" when drowning on Plane of Water
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/trap.c

@@ -4988,6 +4988,9 @@ drown(void)
         /* avoid "drowned in [a] water" */
         if (!strcmp(pool_of_water, "water"))
             pool_of_water = "deep water", gk.killer.format = KILLED_BY;
+        /* avoid "drowned in _a_ limitless water" on Plane of Water */
+        else if (!strcmp(pool_of_water, "limitless water"))
+            gk.killer.format = KILLED_BY;
         Strcpy(gk.killer.name, pool_of_water);
         done(DROWNING);
         /* oops, we're still alive.  better get out of the water. */