multiple FWretract fixes

Keeps Keep FWRETRACT values in terms of millimeters when using M200 for volumetric E units (thanks @daid)
correct feedrate units in comments for M207/M208
Fix bug causing G10/G11 to be broken by using G92 E0 between retract and recover (Slic3r's G10/G11 implementation always does this).

This has been tested and is working using volumetric E units (faked by setting filament diameter in Slic3r to 1.12837916709551) with M200 in start code and Slic3r's "Use firmware retraction" setting.