Also, rework the rc.ifc code that we generate a bit.  We were "falling
back" to 100Mbps if we didn't recognize the speed; now we autoconfig
(like we do for FreeBSD).  We also now try autoconfig for mii-tool
(which of course will never be used again, but gotta be correct).  We
explicitly warn for speeds that mii-tool does not support, instead of
doing something.