beeblink-20201206-213301-cee4410

Add alphabetic drives. Fix various issues.

* any alphanumeric char is now usable as a drive letter

* don't hang on boot when device detection fails; don't hang when
  doing OSARGS Y=0 A=1; close SPOOL/EXEC files on CLOSE #0

* write DFS disks a side at a time. Much faster with Acorn DFS 1.20