AC_Fence: add 10sec manual recovery

This resolves issue #461 by giving the pilot a minimum of 10 seconds to
attempt to manually recover before the autopilot will attempt to retake
control to bring the copter home or land.
Copter: integrate 10sec manual recovery after fence breach
Copter: remove unused variables from fence.pde

saves 2 bytes
Plane: moved auto mode variables to auto_state
GCS_MAVLink: prevent uninitialised bytes being written to a serial port
Replay: fixed ahrs.set_fly_forward() in Replay for copter
Copter: GPS failsafe ensures LAND is pilot-controlled

This resolves issue #880 in which if a GPS failsafe occurred while the
vehicle was already in LAND mode, the LAND would continue to be GPS
controlled LAND instead of pilot-controlled LAND
Copter: RTL alt should not be above fence alt

Resolves issue #435
If the altitude fence is enabled, when the vehicle RTLs it will return
at the lower of the RTL_ALT and the FENCE_ALT_MAX
Copter: land_do_not_use_GPS has no return value
Copter: use get_current_nav_index()
Copter: fence enable/disable via MAVLink