add DEC_KN300 support from NetBSD;

This code makes it possible to run on some of the AlphaServers,
namely AlphaServer 4100 and 1200.

add mcbus(4) and mcpcia(4) to provide support for the system bus and
the MCPCIA-to-PCI bus adapter that can be found in these systems

allow the pci_swiz_bus code to handle variable extent names
to be able to handle more than one mcpcia(4)

"just commit it" deraadt@