implement new means of manipulating buf queues, bufq.

accessed with BUFQ macros, bufq structs support extensible, potentially
changable algorithms and queue formats.  the current default scheme
should support nice priority based queuing, but is missing some vfs_bio.c
support.
only on wd.c for now, other drivers are easy converts.
as a side bonus, this makes the driver code look cleaner.
idea for the name comes from netbsd, but this scheme is incompatible.
thanks to various people for testing.
ok grange@

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