Copter: cleanup some bit if tests to be clearer

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
AP_Common: added BIT_IS_SET() macro

makes bit mask testing safer on PX4

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Copter: use BIT_IF_SET()

this should fix the problem with simple mode on PX4

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>