Use function for combined str/hp loss from poison

Since losestr and losehp calls go together most of the time, this feels
like it probably makes more sense than repeating the killer name/format
twice in a row all over the place.

src/fountain.c

@@ -292,8 +292,8 @@ drinkfountain(void)
                 losehp(rnd(4), "unrefrigerated sip of juice", KILLED_BY_AN);
                 break;
             }
-            losestr(rn1(4, 3), "contaminated water", KILLED_BY);
-            losehp(rnd(10), "contaminated water", KILLED_BY);
+            poison_strdmg(rn1(4, 3), rnd(10), "contaminated water",
+                          KILLED_BY);
             exercise(A_CON, FALSE);
             break;
         case 22: /* Fountain of snakes! */