msg: improve multirotor_motor_limits reporting

msg/multirotor_motor_limits.msg

-uint8 lower_limit	# at least one actuator command has saturated on the lower limit
-uint8 upper_limit	# at least one actuator command has saturated on the upper limit
-uint8 yaw		# yaw limit reached
-uint8 reserved		# reserved
+uint16 saturation_status	# Integer bit mask indicating which axes in the control mixer are saturated
+# 0 - True if any motor is saturated at the upper limit
+# 1 - True if any motor is saturated at the lower limit
+# 2 - True if a positive roll increment will increase motor saturation
+# 3 - True if negative roll increment will increase motor saturation
+# 4 - True if positive pitch increment will increase motor saturation
+# 5 - True if negative pitch increment will increase motor saturation
+# 6 - True if positive yaw increment will increase motor saturation
+# 7 - True if negative yaw increment will increase motor saturation
+# 8 - True if positive thrust increment will increase motor saturation
+# 9 - True if negative thrust increment will increase motor saturation