fix issue #1061 - dipping container into water

Issue reported by loggersviii:  dipping a container into an uncursed
potion of water mentions water getting into the container.  That
happens even when that type of potion hasn't been discovered yet.

Make POT_WATER become discovered if this occurs.  Doesn't apply when
hallucinating where a random liquid is mentioned instead of water.

Fixes #1061

include/decl.h

@@ -613,6 +613,11 @@ struct instance_globals_m {
     /* region.c */
     int max_regions;
 
+    /* trap.c */
+    boolean mentioned_water; /* set to True by water_damage() if it issues
+                              * a message about water; dodip() should make
+                              * POT_WATER should become discovered */
+
     boolean havestate;
     unsigned long magic; /* validate that structure layout is preserved */
 };