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.

serial.c

@@ -71,8 +71,7 @@ void serial_write(uint8_t data) {
 
   // Wait until there is space in the buffer
   while (next_head == tx_buffer_tail) { 
-    protocol_execute_runtime(); // Check for any run-time commands
-    if (sys.abort) { return; } // Bail, if system abort.
+    if (sys.execute & EXEC_RESET) { return; } // Only check for abort to avoid an endless loop.
   }
 
   // Store data and advance head