G43.1/G49 tool length offset installed. Minor bug fix.

- Minor bug fix that caused G92.1 not to work. The mantissa of G92.1
was not computed correctly due to floating point round-off errors and
the use of trunc(). Fixed it by changing the computation with round().

- Installed tool length offsets with G43.1 and G49! True tool length
offsets via G43 are not supported, because these require us to store
tool data that we don’t have space for. But we’ve come up with a good
solution for users that need this. Instead we are strictly using the
dynamic version G43.1 via linuxcnc.org. Visit their website for more
details on the command.

- G43.1 operates by requiring an axis word and value to offset the
configured tool length axis, which can be configured for any axis
(default Z-axis) in config.h. For example, ```G43.1 Z0.5``` will offset
the work coordinates from Z0.0 to Z-0.5.

- G49 will cancel the last tool length offset value and reset it to
zero.

- Tweaked the ‘$#’ parameters report. `Probe` is now `PRB` and a new
value `TLO` states the tool length offset value.