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.

config.h

@@ -85,9 +85,9 @@
 #define N_STARTUP_LINE 2 // Integer (1-3)
 
 // Number of floating decimal points printed by Grbl for certain value types. These settings are 
-// determined by realistic and commonly values observed in CNC machines. For example, position
-// values cannot be less than 0.001mm or 0.0001in, because machines are never more precise than 
-// this. So, there is likely no need to change these, but you can if you need to here.
+// determined by realistic and commonly observed values in CNC machines. For example, position
+// values cannot be less than 0.001mm or 0.0001in, because machines can not be physically more
+// precise this. So, there is likely no need to change these, but you can if you need to here.
 // NOTE: Must be an integer value from 0 to ~4. More than 4 may exhibit round-off errors.
 #define N_DECIMAL_COORDVALUE_INCH 4 // Coordinate or position value in inches
 #define N_DECIMAL_COORDVALUE_MM   3 // Coordinate or position value in mm
@@ -128,6 +128,11 @@
 // step smoothing. See stepper.c for more details on the AMASS system works.
 #define ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING  // Default enabled. Comment to disable.
 
+// Sets which axis the tool length offset is applied. Assumes the spindle is always parallel with 
+// the selected axis with the tool oriented toward the negative direction. In other words, a positive
+// tool length offset value is subtracted from the current location.
+#define TOOL_LENGTH_OFFSET_AXIS Z_AXIS // Default z-axis. Valid values are X_AXIS, Y_AXIS, or Z_AXIS.
+
 // Enables variable spindle output voltage for different RPM values. On the Arduino Uno, the spindle
 // enable pin will output 5V for maximum RPM with 256 intermediate levels and 0V when disabled.
 // NOTE: IMPORTANT for Arduino Unos! When enabled, the Z-limit pin D11 and spindle enable pin D12 switch!