net: if_media for 1000Base-BX BiDi

1000Base-BX uses two wavelengths, commonly 1310nm, 1490nm, 1550nm, or
1590nm, in a Coarse Wavelength Division Multiplexing (CWDM) arrangement
so that a single fiber strand may carry both upstream and downstream.
It is sometimes referred to as BiDi for bi-directional usage of one
fiber.

Optics must be paired such that the RX and TX wavelengths cross over,
with one side often called U(pstream) and the other D(ownstream).

This technology is useful for increasing link density or working around
construction issues, and is also frequently used as a last mile delivery
technology for FTTx.

MFC after: 3 days
Sponsored by: BBOX.io (review/commits)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1518