Fix metric arc tolerence to same value as imperial

Also add an ini file field and allow user variance of arc tolerances
within preset limits

This arose from Linuxcnc forum, with user who could not get CAM generated code for cutting signs to load because of the arc tolerance test.

I found that TOLERANCE_MM had been wrongly set for years to 1/10th of imperial setting instead of
1/25.4th
The fixed metric tolerances have already been pushed into 2.7

Additionally built upon a patch by Dewey Garret to allow setting of arc tolerance in the ini file. 

Contains limits checking against those set in interp_internal.hh, to allow CAM code which contains imprecise or rounded up I, J and K figures in G2 and G3 to run, within sensible bounds.