Add K_CHANGED mode flag for uifc.input()

Used in combination with K_EDIT, if the string isn't actually changed, return
-1 (same as abort/ESC).

This allows the elimination of some save/compare/copy code in SCFG where we
only care to even look at the string if it was changed from its existing
value.

src/uifc/uifc.h

@@ -201,6 +201,7 @@ typedef int64_t uifc_winmode_t;
 #define K_FIND      (1 << 17) /* Don't set the "changes" flag				*/
 #define K_TRIM      (1 << 23) /* Don't allow leading or trailing wsp		*/
 #define K_NOSPACE   (1 << 26) /* Don't allow any wsp chars				*/
+#define K_CHANGED   (1 << 28) /* Return -1 if string was not changed (w/K_EDIT) */
 
 
 /* Extra exit flags */

src/uifc/uifc32.c

@@ -2541,6 +2541,8 @@ int ugetstr(int left, int top, int width, char *outstr, int max, long mode, int
 	{
 		if (!(mode & K_FIND) && strcmp(outstr, str))
 			api->changes = 1;
+		else if (mode & K_CHANGED)
+			j = -1;
 	}
 	else
 	{