include <pci/pcivar.h> without including <sys/devconf.h> and other
drivers include <pci/pcivar.h> before including <sys/devconf.h> if
certain identifiers are defined.
The devconf headers have convoluted interdependencies. <sys/devconf.h>
includes <machine/devconf.h> which includes <pci/pcivar.h>. Most
drivers include <sys/devconf.h> so even isa drivers depend on
<pci/pcivar.h>. For similar reasons, most drivers depend on another
pci header, on an isa header and on two scsi headers.
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