Copter: Activate buzzer when autolanding / buzzer modes

AC3 does autolanding in certain failsafe conditions. For safety reasons and to help locate the copter after a failsafe-induced autolanding, buzzing the buzzer in intervals when in autoland mode would be good. Interval-buzzing is also likely to carry a psychological "do not touch me"-imperative in case, bywalkers find the copter after a failsafe-induced autolanding.

Buzzer could be controlled by a parameter: BUZ_WARN: 0=never (only ARM/DISARM beeps), 1=failsafe autolandings, 2=any autolanding (including manual set LAND mode), 3=any failsafe condition