Improved option for v0.9 GUI compatibility.

- Addressed an issue with backward compatibility with Grbl v0.9-style
GUIs.

- It still may not work due to new data and states coming back from
Grbl v1.1. Regardless, DO NOT TRY TO USE THE COMPATIBILITY MODE UNTIL
THERE IS A REALLY GOOD REASON TO.

- v0.9 GUI compatibility mode will be removed in future versions.
You’ve been warned. It’s highly recommended for GUIs to update to the
new v1.1 interface.

- Compability mode will only fit on an Arduino Uno due to size
increases.

- Removed the REPORT_GUI_MODE compile option since it’s part of the
v1.1 interface standard.

- Updated the documentation to better describe the compatibility mode
build option.

doc/csv/build_option_codes_en_US.csv

@@ -7,7 +7,7 @@
 "Z","Homing force origin","Enabled"
 "H","Homing single axis commands","Enabled"
 "L","Two limit switches on axis","Enabled"
-"R","Classic status reporting","Enabled"
+"R","Classic compatibility mode","Enabled"
 "*","Restore all EEPROM command","Disabled"
 "$","Restore EEPROM `$` settings command","Disabled"
 "#","Restore EEPROM parameter data command","Disabled"