Improved fix for rare lowering feed/rapid override bug.

[fix] In the previous hot fix, there was still (rarer) potential for
very small floating point errors incorrectly handle an override
deceleration and crash Grbl. Re-factored the if-then statement in terms
of speed changes, rather than distance, to completely eliminate the
issue.

Hot fix for rare lowering feed override bug.

[fix] Squashed a very rare bug when lowering the feedrate (or rapid) override. When in the very strict set of circumstances with acceleration settings, override step size, and current speed, an internal calculation would cause Grbl to crash. The fix was an overlooked equality statement that should have been a less than or equal, not a less than.

Clean up and new streaming script check-mode feature.

[new] The stream.py streaming script now has a check-mode option, where it will place Grbl in $C check mode automatically and then stream the g-code program. It's a very fast way to check if the g-code program has any errors.

[fix] The debug variable was not initialized if the debug option was enabled in config.h

[fix] Updated error_codes CSV file to the same format as the others.

Additional build info in the $I printout.

- [new] Added total available planner buffer blocks (15 shown, but
there are 16. one is used by the ring buffer and to execute system
motions) and serial RX buffer bytes. This information is useful for
GUIs to setup and optimize their streaming protocols easily.

[doc] Updated the interface document to reflect the change.

Tool number bug fix. Updated documentation.

- [fix] Tool numbers were not being tracked and reported correctly. Now
shows tool number values in $G when programmed.

- [fix] Added a max tool number value check to the g-code parser.

- [doc] Added a new error code for invalid tool number. Updated CSV and
interface documents.

- [doc] Added a implementation note for buffer state in status reports.
Don’t rely on this data for streaming.

Fixed homing fail alarm handling. Re-integrated software debouncing.

- [bug] Fixed a homing fail issue, where the alarm was not being set
right, not cleared correctly. It would report the wrong code and enter
an infinite alarm loop. This was due to how alarm codes were altered a
while back. Now updated and fixed to show the right codes.

- [feature] Re-installed optional software debouncing for hard limit
switches. By request.

Removed classic GUI interface. Fixed typo with line number support.

- [config] Permanently removed classic GUI interface support. This
unintentionally created a problem where some users/GUI devs used this
compatibility mode and did not update to the new interface. So, there
were two interfaces in use, rather than just one like it was intended.
This removal should help everyone by forcing all GUIs to update and
updated GUI not having to support yet another interface.

- Fixed typo with line number support in jog mode.