Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make appropriate changes to use devfs.
Add a reference to the fd_size variable to set floppy size.

Bring the new script into -current. Still untested here, but there
is a high chance that they work modulo minor things.