Next phase in the PCI subsystem cleanup.

Next phase in the PCI subsystem cleanup.

 - Move PCI core code to dev/pci.
 - Split bridge code out into separate modules.
 - Remove the descriptive strings from the bridge drivers.  If you
   want to know what a device is, use pciconf.  Add support for
   broadly identifying devices based on class/subclass, and for
   parsing a preloaded device identification database so that if
   you want to waste the memory, you can identify *anything* we know
   about.
 - Remove machine-dependant code from the core PCI code.  APIC interrupt
   mapping is performed by shadowing the intline register in machine-
   dependant code.
 - Bring interrupt routing support to the Alpha
   (although many platforms don't yet support routing or mapping
   interrupts entirely correctly).  This resulted in spamming
   <sys/bus.h> into more places than it really should have gone.
 - Put sys/dev on the kernel/modules include path.  This avoids
   having to change *all* the pci*.h includes.