[9fans] jukebox configuration changes

[Geoff Collyer <geoff@collyer.net>]

I've just modified the file server kernel to cope with different
DSIZEs (optical disc sizes) per jukebox and even per disc.  It's now
also possible to change FIXEDSIZES in /sys/src/fs/*/9*.c rather than
having to edit dev/juke.c before compiling each distinct kernel.

The way it used to work was that DSIZE was the size in RBUFSIZE-byte
blocks of each optical disc *side*.  However, it is possible to use
discs of different sizes within a single jukebox: you set FIXEDSIZES
to 0 and pay with a delay at each boot while the file server pulls
each disc side in and asks the drive how big it is.  It's probably
wiser to use discs of the same size and set FIXEDSIZES to 1, which
makes the file server just look at one disc and assume that all the
sides are the size of its.  So DSIZE was only used for computing the
statistics that "statw" prints on the console, for working out sizes
in the cache save and restore commands, and in the implementation of
"check touch".  Those operations could be led astray if DSIZE wasn't
the size in Plan 9 blocks of all the optical discs in all jukeboxes on
a given file server.  If one wanted to put a jukebox with 5.2GB discs
and one with 2.6GB discs on the same file server, it wouldn't work
perfectly.

DSIZE is gone.  It's now possible to mix jukeboxes with discs of
different sizes, and even, if you're willing to put up with the
delays, to use discs of different sizes within a single jukebox, and
everything should now work right.

The whole thing compiles but I want to test it tomorrow (after the
third jukebox arrives) before I offer the code to anyone else.