- Rename PCIx_HEADERTYPE* to PCIx_HDRTYPE* so the constants aren't so long.

- Rename PCIx_HEADERTYPE* to PCIx_HDRTYPE* so the constants aren't so long.
- Add a new PCIM_HDRTYPE constant for the field in PCIR_HDRTYPE that holds
  the header type.
- Replace several magic numbers with appropriate constants for the header
  type register and a couple of PCI_FUNCMAX.
- Merge to amd64 the fix to the i386 bridge code to skip devices with
  unknown header types.

Requested by:	imp (1, 2)