Add "user canceled" as extended command return value

Instead of returning ECMD_OK, the commands now return ECMD_CANCEL
when user declined to pick a direction or an object to act on.

Note that this can be ORed with ECMD_TIME, if the command still
took a turn.

For now this has no gameplay meaning.

src/pager.c

@@ -1739,7 +1739,7 @@ doidtrap(void)
     int x, y, tt, glyph;
 
     if (!getdir("^"))
-        return ECMD_OK;
+        return ECMD_CANCEL;
     x = u.ux + u.dx;
     y = u.uy + u.dy;