M26 does not work at start of print

[richfelker]

The M26 (set file position) command works if there is an active print that has been paused, but does nothing (subsequent M24 starts at position zero regardless, and M27 shows position zero) if used after selecting a file with M23 but before starting the print with M24. This seems to be caused by some discrepancy between self.next_file_position (which M26 sets via set_file_position) and self.file_position, but I don't understand the async logic well enough to fix it.

This makes it gratuitously difficult to recover prints after error.