added grbl commands when in grbl_mode (set in config with grbl_mode true)

added grbl commands when in grbl_mode (set in config with grbl_mode true)
 $H does homing cycle
 $G returns gcod state
 $X resets Alarm state
 $# outputs WCS state

error messages are more grbl like "error:message"
G28 in grbl mode is goto predefined position (not quite implemented yet)
Probing with G38.2/3 now returns grbl messages when in grbl mode, and enters Alarm if requested