AC_Circle: add get_closest_point_on_circle

init_start_angle method added to use current heading or position to
decide on initial start angle
Copter: integrate change in AC_Circle init

Removed init_loiter_target when vehicle landed.  We should add some
initialisation of the circle controller when landed.
Copter: Loiter_Turns moves to edge of circle if location provided
Copter: remove unused circle_desired_rotations

desired num rotations is already held in command so we save 1byte of RAM