Make extended commands return defined flags

Instead of returning 0 or 1, we'll now use ECMD_OK or ECMD_TURN.
These have the same meaning as the hardcoded numbers; ECMD_TURN
means the command uses a turn.

In future, could add eg. a flag denoting "user cancelled command"
or "command failed", and should clear eg. the cmdq.

Mostly this was simply replacing return values with the defines
in the extended commands, so hopefully I didn't break anything.

src/weapon.c

@@ -1289,7 +1289,7 @@ enhance_weapon_skill(void)
             }
         }
     } while (speedy && n > 0);
-    return 0;
+    return ECMD_OK;
 }
 
 /*