G03 Full circle did not work (G02 is working correct)

If [XYZ]_HOME_POS and [XYZ]_MIN_POS aren't 0, these corrections are
wrong.  Use the same logic as in Marlin.pde:prepare_move: ie, clamp to
[XYZ]_{MIN,MAX}_POS.

While we're here, put this cut-and-paste code in a function
clamp_to_software_endstops.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Fixed small bug in arcs
M105 prints now actual_temp/target_temp

if (millimeters_of_travel == 0.0) => if (millimeters_of_travel < 0.001)
(thanks mooselake)

Add serial checkRx in stepper ISR.
Copied HardwareSerial to MarlinSerial (Needed for checkRx).