Expanded M302 to allow setting the temp

Cold extrusion prevention can be very useful, but the temperature limit differs quite a lot between ABS and PLA, and sometimes even between colours.

M302 is updated so it allows you to change the temperature. M302 S170 will set the cold extrusion temperature limit to 170C instead of disabling it.

M302 without the S-parameter will set the limit to 0C, effectively disabling cold extrusion prevention, making it backwards compatible with old G-Code.