groundwork: u.uinwater manipulation

Toggle u.uinwater on or off in just one place.

src/trap.c

@@ -3805,7 +3805,7 @@ drown()
             placebc();
         }
         vision_recalc(2); /* unsee old position */
-        u.uinwater = 1;
+        set_uinwater(1); /* u.uinwater = 1 */
         under_water(1);
         g.vision_full_recalc = 1;
         return FALSE;
@@ -3871,7 +3871,7 @@ drown()
         /* still too much weight */
         pline("But in vain.");
     }
-    u.uinwater = 1;
+    set_uinwater(1); /* u.uinwater = 1 */
     You("drown.");
     for (i = 0; i < 5; i++) { /* arbitrary number of loops */
         /* killer format and name are reconstructed every iteration
@@ -3890,7 +3890,7 @@ drown()
         pline("You're still drowning.");
     }
     if (u.uinwater) {
-        u.uinwater = 0;
+        set_uinwater(0); /* u.uinwater = 0 */
         You("find yourself back %s.",
             Is_waterlevel(&u.uz) ? "in an air bubble" : "on land");
     }