Guided-Spline has been replaced by Guided-PosVel

Also set auto-yaw-mode to yaw-hold only when posvel controller is
started instead of on every iteration.
added GUIDED_POSVEL_TIMEOUT_MS definition.
removed deprecated trigger_xy call.
Also use wp_nav.get_speed_xy in place of get_speed_param

no functional change

This allows the GCS to display to the user where the vehicle is flying
to in RTL and Guided flight modes

This command was not implemented fully removing this code resolves a
compiler warning

This avoids the chance of an arithmetic exception in cases where the
position is moved too far