Copter: increase LOITER_RATE_IMAX to 1000

This allows us to load up the I term with the vehicle's current
acceleration during mode transitions
AC_AttControl: lean_angle_max accessor fn const
AC_WPNav: add LOIT_JERK parameter

Limit accel output from loiter controller.
Call new pos_control.init_xy_controller when loiter starts
Remove sudden stop when pilot requested acceleration is zero
AC_PosControl: lean_angles_to_accel added for smooth initialisation

init_xy_controller also added to capture initialisation all in one place