Add a new driver for the AMD PCnet/FAST, FAST+ and Home PCI adapters.

Add a new driver for the AMD PCnet/FAST, FAST+ and Home PCI adapters.
Previously, these cards were supported by the lnc driver (and they
still are, but the pcn driver will claim them first), which is fine
except the lnc driver runs them in 16-bit LANCE compatibility mode.
The pcn driver runs these chips in 32-bit mode and uses the RX alignment
feature to achieve zero-copy receive. (Which puts it in the same
class as the xl, fxp and tl chipsets.) This driver is also MI, so it
will work on the x86 and alpha platforms. (The lnc driver is still
needed to support non-PCI cards. At some point, I'll need to newbusify
it so that it too will me MI.)

The Am79c978 HomePNA adapter is also supported.