See also https://github.com/MarlinFirmware/Marlin/pull/4370#issuecomment-234713680

・Update description of M111

・Remove unneeded PSTR()

Unify run_z_probe
Add double touch for DELTAs.
Introduce Z_PROBE_SPEED_FAST and Z_PROBE_SPEED_SLOW
  defaulting to homing_feedrate_mm_m[Z_AXIS] and homing_feedrate_mm_m[Z_AXIS]/2

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.