Implement support for selecting SGMII or SerDes mode depending on the

Implement support for selecting SGMII or SerDes mode depending on the
plugged-in SFP transceiver and for reading out transceiver information
via ifconfig(8).  To read from the SFP, we need to let the card issue
I2C transfers.  Additionally we need I2C to read/write to the PHY when
MDIO is not available.  Depending on the SFP's supported media types
we can decide which mode to use.

This fixes hardware-initialization and link-up problems with some em(4)
Fiber NIC and SFP combinations.

Tested by dlg@ and been in snaps for quite a while
ok dlg@ jmatthew@