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/markdown/interface.md

@@ -311,7 +311,7 @@ Feedback messages provide non-critical information on what Grbl is doing, what i
 | **`Z`** | Homing force origin enabled |
 | **`H`** | Homing single axis enabled |
 | **`L`** | Two limit switches on axis enabled |
-| **`R`** | Classic status reporting enabled |
+| **`R`** | Classic compatiblity mode enabled |
 | **`*`** | Restore all EEPROM disabled |
 | **`$`** | Restore EEPROM `$` settings disabled |
 | **`#`** | Restore EEPROM parameter data disabled |