#9 WARMBOOT implementation

 - implemented warm boot from msxdos.sys that calculates command.com
checksum and loads it if necessary
 - this changes bootstrap procedure: it now ends with rst 0
 - programs can now return to shell and it appears to be reloading well
 - but death after first command still happens: the prompt appears, but
the first key press tends to kill cause warmboot, or kill it to death —
investigate readline?