First step towards plugging the "pccard is trying to map into a BIOS

First step towards plugging the "pccard is trying to map into a BIOS
region for CIS reading" problem:

Use bus_alloc_resource to get the memory that we'll be using.  Also
has the benefit of doing usage checking as well.  This gets rid of the
ugly kludge that we had before for mapping pmem to vmem.

Second, move PIOCSRESOURCE to its own routine and make it conform more
to style(9) in the process.