Copter: only check hdop when circular fence enabled
Copter: reduce sonar sensitivity to spikes
AC_Fence: add safety margin parameter

This will allow auto modes to proactively avoid breaching the fence
Copter: Loiter and AltHold won't breach alt fence
Copter: add ch7/ch8 Auto and Land option

CH7/CH8 high initiates AUTO mode, low initiates Land.  Ideally high
would be a new take-off flight mode so this may need some rework.