WUtil: Changed declaration of 'wusergnusteppath' to return a CONST string

According to the way its value is being used everywhere, that is
what would be expected, so let's make it official.

Please note that this may introduce warnings on user code using
this function ("...discard const...") but that's an opportunity
for them to check that their code is not doing anything wrong.

WINGs/WINGs/WUtil.h

@@ -816,7 +816,7 @@ Bool WMWritePropListToFile(WMPropList *plist, const char *path);
 /* ---[ WINGs/userdefaults.c ]-------------------------------------------- */
 
 /* don't free the returned string */
-char* wusergnusteppath(void);
+const char* wusergnusteppath(void);
 
 /* Free the returned string when you no longer need it */
 char* wdefaultspathfordomain(const char *domain);

WINGs/userdefaults.c

@@ -46,7 +46,7 @@ extern char *WMGetApplicationName();
 #define UD_SYNC_INTERVAL	2000
 #endif
 
-char *wusergnusteppath()
+const char *wusergnusteppath()
 {
 	static char *path = NULL;
 	char *gspath;