AC_PosControl: remove reset_I_xy

lean_angles_to_accel is now used to load the position rate I terms based
on vehicle's initial roll and pitch angle
WPNav: add wp_and_spline_init to set speeds an init pos controller
Copter: RTL calls wp_and_spline_init
Copter: AUTO and Guided call wp_and_spline_init
Copter: reduce Circle init twitch by using stopping point
Copter: set pos controller speeds before circle init

This ensures stopping point is calculated correctly