New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

pegs.c

@@ -848,7 +848,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 	    ui->dx = x;
 	    ui->dy = y;
             ui->cur_visible = ui->cur_jumping = 0;
-	    return "";		       /* ui modified */
+	    return UI_UPDATE;
 	}
     } else if (button == LEFT_DRAG && ui->dragging) {
 	/*
@@ -856,7 +856,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 	 */
 	ui->dx = x;
 	ui->dy = y;
-	return "";		       /* ui modified */
+	return UI_UPDATE;
     } else if (button == LEFT_RELEASE && ui->dragging) {
 	int tx, ty, dx, dy;
 
@@ -868,18 +868,18 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 	tx = FROMCOORD(x);
 	ty = FROMCOORD(y);
 	if (tx < 0 || tx >= w || ty < 0 || ty >= h)
-	    return "";		       /* target out of range */
+	    return UI_UPDATE;	       /* target out of range */
 	dx = tx - ui->sx;
 	dy = ty - ui->sy;
 	if (max(abs(dx),abs(dy)) != 2 || min(abs(dx),abs(dy)) != 0)
-	    return "";		       /* move length was wrong */
+	    return UI_UPDATE;	       /* move length was wrong */
 	dx /= 2;
 	dy /= 2;
 
 	if (state->grid[ty*w+tx] != GRID_HOLE ||
 	    state->grid[(ty-dy)*w+(tx-dx)] != GRID_PEG ||
 	    state->grid[ui->sy*w+ui->sx] != GRID_PEG)
-	    return "";		       /* grid contents were invalid */
+	    return UI_UPDATE;	       /* grid contents were invalid */
 
 	/*
 	 * We have a valid move. Encode it simply as source and
@@ -899,7 +899,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                 ui->cur_x = cx;
                 ui->cur_y = cy;
             }
-            return "";
+            return UI_UPDATE;
         } else {
             int dx, dy, mx, my, jx, jy;
 
@@ -922,21 +922,21 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                 ui->cur_x = jx; ui->cur_y = jy;
                 return dupstr(buf);
             }
-            return "";
+            return UI_UPDATE;
         }
     } else if (IS_CURSOR_SELECT(button)) {
         if (!ui->cur_visible) {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
         if (ui->cur_jumping) {
             ui->cur_jumping = 0;
-            return "";
+            return UI_UPDATE;
         }
         if (state->grid[ui->cur_y*w+ui->cur_x] == GRID_PEG) {
             /* cursor is on peg: next arrow-move wil jump. */
             ui->cur_jumping = 1;
-            return "";
+            return UI_UPDATE;
         }
         return NULL;
     }