Extended position reporting with both home and work coordinates. Home position now retained after reset. Other minor changes/fixes.

- Grbl now tracks both home and work (G92) coordinate systems and does
live updates when G92 is called.
- Rudimentary home and work position status reporting. Works but still
under major construction.
- Updated the main streaming script. Has a disabled periodic timer for
querying status reports, disabled only because the Python timer doesn't
consistently restart after the script exits. Add here only for user
testing.
- Fixed a bug to prevent an endless serial_write loop during status
reports.
- Refactored the planner variables to make it more clear what they are
and make it easier for clear them.

limits.c

@@ -27,7 +27,8 @@
 #include "motion_control.h"
 #include "planner.h"
 
-// TODO: Deprecated. Need to update for new version.
+// TODO: Deprecated. Need to update for new version. Sys.position now tracks position relative
+// to the home position. Limits should update this vector directly.
 
 void limits_init() {
   LIMIT_DDR &= ~(LIMIT_MASK);