[9fans] fs kernel ether driver update

[geoff@collyer.net]

I just put up a second file server for testing.  This one has a
built-in SiS ethernet and I was able to exercise it; it seems to be
fine with the current driver.  (I did have to change the boot programs
to recognise the SiS interface, and those changes have been submitted
as a patch, and still require one to specify ea= in plan9.ini for SiS
interfaces.)

I've also exercised an i82557 with a new driver.  It seems to be fine,
so I've copied that driver to
/n/sources/plan9/sys/src/fs/pc/ether82557.c.  Like ether83815.c,
ether8139.c, etherdp83820.c, and etherigbe.c, this new 82557 driver
will work unmodified in cpu kernels that have /sys/src/9/pc/compat.h.

There are more driver updates coming as I get the chance to verify
that the new drivers work correctly.