MFC: r265263

Make this manpage ready for hardware notes generation, add more Xrefs and perform
general cleanup.

In particular, don't claim rsu(4) devices can be configured with hostname(1)...
MFC: r257408 by kevlo

Add manpage for urtwnfw, the Realtek RTL8188CU/RTL8192CU firmware
module.  Also fix a few nits in urtwn.4.
MFC: r265264, r265270

Add rsu(4) and urtwn(4) to the hardware notes.