Initialize all outgoing vehicle_command_ack_s and vehicle_command_s

This will initialize those structs with zero in all fields not set
and all fields set will only be change once to the final value not
wasting CPU time zeroing it.

This will guarantee that no non-unitialized structs will have
a trash value on from_external causing it to be sent to the
MAVLink channel without need it.

src/platforms/posix/tests/muorb/muorb_test_example.h

@@ -57,6 +57,6 @@ private:
 	int DefaultTest();
 	int PingPongTest();
 	struct esc_status_s m_esc_status;
-	struct vehicle_command_s m_vc;
+	struct vehicle_command_s m_vc = {};
 
 };

src/platforms/qurt/tests/muorb/muorb_test_example.h

@@ -54,6 +54,6 @@ private:
 	int uSleepTest();
 
 	struct esc_status_s m_esc_status;
-	struct vehicle_command_s m_vc;
+	struct vehicle_command_s m_vc = {};
 
 };