Fixed a very rare but critical bug when reducing override rates.

- See main branch commit log for details.

Syncing mainstream v1.1f

- See details in mainstream commits from 1/14/17 to 7/17/17.

Minor bug fix to certain safety conditions.

- Grbl would become unresponsive, if a safety door is reset when active
and then homed immediately after. A system variable was not properly
restored. Now fixed.