groundwork: u.uinwater manipulation

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

include/extern.h

@@ -897,6 +897,7 @@ E void NDECL(domove);
 E boolean NDECL(overexertion);
 E void NDECL(invocation_message);
 E void NDECL(switch_terrain);
+E void FDECL(set_uinwater, (int));
 E boolean FDECL(pooleffects, (BOOLEAN_P));
 E void FDECL(spoteffects, (BOOLEAN_P));
 E char *FDECL(in_rooms, (XCHAR_P, XCHAR_P, int));