hangup vs u.uinwater again (trunk only)

Redo the fix that prevents hangup from putting hero on top of water
if it occurs during magic mapping or object/gold/trap detection.  Instead
of copying u.uinwater into another field in struct u so that it can be
reset during restore, copy it into a new field in struct iflags and reset
it during save so that no fixup upon restore is needed.

include/flag.h

@@ -306,6 +306,11 @@ struct instance_flags {
 #define MAX_ALTKEYHANDLER 25
 	char	 altkeyhandler[MAX_ALTKEYHANDLER];
 #endif
+	/* copies of values in struct u, used during detection when the
+	   originals are temporarily cleared; kept here rather than
+	   locally so that they can be restored during a hangup save */
+	Bitfield(save_uinwater,1);
+	Bitfield(save_uburied,1);
 };
 
 /*