Two 'dynamic driver' features in FS:
. When drivers disappear that have pending select()s, wake up
those user processes with EAGAIN so that they can retry their
select() and won't hang forever on it.
. When drivers re-appear and are mapped into the dmap, run through
the list of mounted filesystems and re-dev_open() every one (for
partition tables and such). This can't happen before the driver
has exec()ced itself, so processes that have fork()ed but not
exec()ced yet are marked as DMAP_BABY in the dmap table if they
are dmapped before they are execced. If that happens, the above
procedure happens after the exec(). If the exec() happens before
the dmapping, it (the dev_open()ing) happens right away.