Convert xl(4) to new media and miibus KPI.

xl(4) supports a zillion of varieties of 3Com NICs, so it is quite complex.
Some NICs have miibus(4), and some not. Some miibus NICs have extra mediae,
that are added from the NIC driver on top of phys mediae.

So, unlike in other NIC drivers, the mediae list isn't static, but allocated
per-softc. Actually it is possible to build a set of static mediae arrays,
matching every possible NIC, but such approach would require testing on every
possible NIC.

Functional changes:
o Since not tested on miibus(4) based xl(4), be conservative and

don't declare IFCAP_LINKSTATE.

Tested: 3Com 3c900B-COMBO Etherlink XL, only UTP tested
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