AC_WPNav: add reference to AC_PosControl

Also remove requirement to pass in inertial nav position and velocity to
init_loiter method
AC_PosControl: add horizontal pos control

Not yet working completely (or even compiling completely)
AC_AttControl: accessor for lean angle max
Copter: integrated simpler init loiter from WP_Nav
Copter: integrate add ref to AC_Pos_Control from WP_Nav