Copter: reset battery_fs after dis/rearming

A user pointed out that one the battery_failsafe was trigged, and you landed/disarmed, you were free to take off, and the fs would not be trigged again. 
http://ardupilot.com/forum/viewtopic.php?t=9590&p=22905#p22905

- I added some code to reset it upon arming.