add tests for t0 handling

Thanks to Chris Radek for the workaround for a +0/-0 float rounding
error instablility in the test results.
fix a t0 bug in Interp::convert_setup_tool

This would accidentally update the tool-related numbered parameters with
the info for T0, when using G10 to update a tool, while no tool is in
the spindle, on random-tc machines.

Most of the time this is probably ok, since by convention T0 is usually
used as the "no tool" too, in which case it has 0 for all its TLOs.
But it's still a bug: #5400 (current tool) would be wrong even in this
case, and the TLO params would be wrong if T0 was used for a real tool.
docs: note that G10 cannot set TLO for tool 0
docs: update T-word documentation to match reality

The previous version did not adequately describe the operation of T0 with
respect to random vs nonrandom toolchangers, and incorrectly related tool
number and pocket number.
docs: add an anchor for the Tool Changers section
docs: clean up the G43 section a bit
docs: note funny behavior of G41/G42 D0
pncconf -fix a missing signal in openloop test.

Dir was always required even when not needed