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Distinguish MOVE_UNUSED from MOVE_NO_EFFECT in Dominosa

The only tricky bit is whether clicking precisely on the diagonal of a
square (which never has any effect on the game) is MOVE_UNUSED or
MOVE_NO_EFFECT.  I decided that having single-pixel lines in the middle
of the grid causing events to be passed back to the environment would be
very confusing, so they're MOVE_NO_EFFECT.  Clicking entirely outside
the grid, on the other hand, returns MOVE_UNUSED.

--- a/dominosa.c

+++ b/dominosa.c

@@ -2799,7 +2799,7 @@

         int d1, d2;

         if (tx < 0 || tx >= w || ty < 0 || ty >= h)

-            return NULL;

+            return MOVE_UNUSED;

         /*

          * Now we know which square the click was in, decide which

@@ -2817,7 +2817,7 @@

         else if (abs(dy) > abs(dx) && dy > 0 && ty+1 < h)

             d1 = t, d2 = t + w;        /* clicked in top half of domino */

         else

-            return NULL;

+            return MOVE_NO_EFFECT;     /* clicked precisely on a diagonal */

         /*

          * We can't mark an edge next to any domino.

@@ -2824,7 +2824,7 @@

          */

         if (button == RIGHT_BUTTON &&

             (state->grid[d1] != d1 || state->grid[d2] != d2))

-            return NULL;

+            return MOVE_NO_EFFECT;

         ui->cur_visible = false;

         sprintf(buf, "%c%d,%d", (int)(button == RIGHT_BUTTON ? 'E' : 'D'), d1, d2);

@@ -2839,7 +2839,7 @@

         int d1, d2;

 	if (!((ui->cur_x ^ ui->cur_y) & 1))

-	    return NULL;	       /* must have exactly one dimension odd */

+            return MOVE_NO_EFFECT; /* must have exactly one dimension odd */

 	d1 = (ui->cur_y / 2) * w + (ui->cur_x / 2);

 	d2 = ((ui->cur_y+1) / 2) * w + ((ui->cur_x+1) / 2);

@@ -2848,7 +2848,7 @@

          */

         if (button == CURSOR_SELECT2 &&

             (state->grid[d1] != d1 || state->grid[d2] != d2))

-            return NULL;

+            return MOVE_NO_EFFECT;

         sprintf(buf, "%c%d,%d", (int)(button == CURSOR_SELECT2 ? 'E' : 'D'), d1, d2);

         return dupstr(buf);

@@ -2855,7 +2855,7 @@

     } else if (isdigit(button)) {

         int n = state->params.n, num = button - '0';

         if (num > n) {

-            return NULL;

+            return MOVE_UNUSED;

         } else if (ui->highlight_1 == num) {

             ui->highlight_1 = -1;

         } else if (ui->highlight_2 == num) {

@@ -2865,12 +2865,12 @@

         } else if (ui->highlight_2 == -1) {

             ui->highlight_2 = num;

         } else {

-            return NULL;

+            return MOVE_NO_EFFECT;

         }

         return MOVE_UI_UPDATE;

     }

-    return NULL;

+    return MOVE_UNUSED;

 }

 static game_state *execute_move(const game_state *state, const char *move)