nbaf: drain playfield balls when flippers off

On WPC games, solenoid 31 indicates whether the flippers are enabled or
not.  If the ball isn't held (e.g., Crazy Bob's) and in the "Playfield"
state, we can automatically drain it.  Helpful for halftime and end of
game during Championship Link.

src/wpc/sims/wpc/prelim/nbaf.c

@@ -280,6 +280,14 @@ static sim_tState nbaf_stateDef[] = {
 
 static int nbaf_handleBallState(sim_tBallStatus *ball, int *inports) {
   switch (ball->state) {
+  case stFree:
+      // Automatically drain any balls on the playfield (and not held somewhere)
+      // when the flippers are turned off.
+      if (!core_getSol(31)) {
+          return setState(stDrain, 5);
+      }
+      break;
+
   case stShooter1:
       if (core_getSol(sPassRight1)) {
           return setState(stShooter2, 5);