Introduce line_to_axis_pos(axis_codes axis, float where, float feed_rate = 0.0)

and use it in `homeaxis()` instead of `do_blocking_move_to_axis_pos()`.
`do_blocking_move_to_axis_pos` was wrong because it performed subdivided, delta-corrected moves for x- and y-axis.

The first common move for delta homing is like quick_home but for 3 towers.

Fix two warnings.