Fix a bug in the last commit. 4.x improperly tries to add the children

Fix a bug in the last commit.  4.x improperly tries to add the children
of a PCI bus during the probe phase.  This doesn't mix well with 5.x's
bus code which adds the children in the attach phase.  Clean up
pci_probe().  The code, unfortunately, is still quite fragile.

Cleanup pci_read_device().

Bring in pcib_*() routines from 5.x into pcisupport.c.  Add support for
bridges which do not support particular PCI mapping types (from FreeBSD 5.x).

Fix a bug in the pcib_route_interrupt() method.