APMrover2 : correct arming and failsafe in skid steer mode

Commit 3636b53#diff-e22a85a55f71f1b9b2d3f293dea61368 introduce arming for rover. But in skid steering mode it was only applied on throttle and the rover still pivot.
The patch also correct rover behaviour in case of failsafe or loitering.

APMrover2/Steering.cpp

@@ -86,6 +86,10 @@ void Rover::calc_throttle(float target_speed) {
     // then set the throttle to minimum
     if (!auto_check_trigger() || ((loiter_time > 0) && (control_mode == AUTO))) {
         channel_throttle->set_servo_out(g.throttle_min.get());
+        // Stop rotation in case of loitering and skid steering
+        if (g.skid_steer_out) {
+            channel_steer->set_servo_out(0);
+        }
         return;
     }
 
@@ -223,6 +227,10 @@ void Rover::set_servos(void) {
         if (failsafe.bits & FAILSAFE_EVENT_THROTTLE) {
             // suppress throttle if in failsafe and manual
             channel_throttle->set_radio_out(channel_throttle->get_radio_trim());
+            // suppress steer if in failsafe and manual and skid steer mode
+            if (g.skid_steer_out) {
+                channel_steer->set_radio_out(channel_steer->get_radio_trim());
+            }
         }
     } else {
         channel_steer->calc_pwm();
@@ -239,10 +247,18 @@ void Rover::set_servos(void) {
         if ((failsafe.bits & FAILSAFE_EVENT_THROTTLE) && control_mode < AUTO) {
             // suppress throttle if in failsafe
             channel_throttle->set_servo_out(0);
+            // suppress steer if in failsafe and skid steer mode
+            if (g.skid_steer_out) {
+                channel_steer->set_servo_out(0);
+            }
         }
 
         if (!hal.util->get_soft_armed()) {
             channel_throttle->set_servo_out(0);
+            // suppress steer if in failsafe and skid steer mode
+            if (g.skid_steer_out) {
+                channel_steer->set_servo_out(0);
+            }
         }
 
         // convert 0 to 100% into PWM
@@ -285,11 +301,17 @@ void Rover::set_servos(void) {
 
         case AP_Arming::YES_ZERO_PWM:
             channel_throttle->set_radio_out(0);
+            if (g.skid_steer_out) {
+                channel_steer->set_radio_out(0);
+            }
             break;
 
         case AP_Arming::YES_MIN_PWM:
         default:
             channel_throttle->set_radio_out(channel_throttle->get_radio_trim());
+            if (g.skid_steer_out) {
+                channel_steer->set_radio_out(channel_steer->get_radio_trim());
+            }
             break;
         }
     }