Hard limits, homing direction, pull-off limits after homing, status reports in mm or inches, system alarm, and more.

- Thank you statement added for Alden Hart of Synthetos.

- Hard limits option added, which also works with homing by pulling off
the switches to help prevent unintended triggering. Hard limits use a
interrupt to sense a falling edge pin change and immediately go into
alarm mode, which stops everything and forces the user to issue a reset
(Ctrl-x) or reboot.

- Auto cycle start now a configuration option.

- Alarm mode: A new method to kill all Grbl processes in the event of
something catastrophic or potentially catastropic. Just works with hard
limits for now, but will be expanded to include g-code errors (most
likely) and other events.

- Updated status reports to be configurable in inches or mm mode. Much
more to do here, but this is the first step.

- New settings: auto cycle start, hard limit enable, homing direction
mask (which works the same as the stepper mask), homing pulloff
distance (or distance traveled from homed machine zero to prevent
accidental limit trip).

- Minor memory liberation and calculation speed ups.

stepper.c

@@ -113,8 +113,8 @@ void st_go_idle()
   // Force stepper dwell to lock axes for a defined amount of time to ensure the axes come to a complete
   // stop and not drift from residual inertial forces at the end of the last movement.
   delay_ms(settings.stepper_idle_lock_time);
-  // Disable steppers by setting stepper disable
-  if (settings.stepper_idle_lock_time != 0xff) {
+  // Disable steppers only upon system alarm activated or by user setting to keep enabled.
+  if ((settings.stepper_idle_lock_time != 0xff) || bit_istrue(sys.execute,EXEC_ALARM)) {
     STEPPERS_DISABLE_PORT |= (1<<STEPPERS_DISABLE_BIT);
   }
 }