AP_Arming: add do_disarm_checks boolean to disarm call

this creates symmetry between arming and disarming, at least as far as
the top-level arm() and disarm() calls are concerned.

libraries/AP_Arming/AP_Arming.cpp

@@ -1204,7 +1204,7 @@ bool AP_Arming::arm(AP_Arming::Method method, const bool do_arming_checks)
 }
 
 //returns true if disarming occurred successfully
-bool AP_Arming::disarm(const AP_Arming::Method method)
+bool AP_Arming::disarm(const AP_Arming::Method method, bool do_disarm_checks)
 {
     if (!armed) { // already disarmed
         return false;

libraries/AP_Arming/AP_Arming.h

@@ -86,7 +86,7 @@ public:
     // these functions should not be used by Copter which holds the armed state in the motors library
     Required arming_required();
     virtual bool arm(AP_Arming::Method method, bool do_arming_checks=true);
-    virtual bool disarm(AP_Arming::Method method);
+    virtual bool disarm(AP_Arming::Method method, bool do_disarm_checks=true);
     bool is_armed();
 
     // get bitmask of enabled checks