ifconfig.8: Add capability codes descriptions from handbook

[0mp]

Summary

We document capability codes (the CAPS field of "ifconfig wlan0 scan")
in both ifconfig(8) and the handbook. The list is more complete in the
manual page, while the descriptions of individual capabilities are more
detailed in the handbook.

In order to reduce content duplication and bit rot, let's move
handbook's details to the manual page and reference the manual page
whenever necessary.

MFC after: 2 weeks

[0mp]

Related handbook change: https://reviews.freebsd.org/D34663

[debdrup]

Syntax looks good, and it matches what I remember of my CCIE Wireless courses.

Do we want to mention WEP and TKIP when both are considered deprecated because of security considerations, though?

[0mp]

I'd leave it as it is. As it is now, this is just a copy of the content from the handbook. If desired, we can iterate and clean up deprecated bits later on.

[pauamma_gundo.com]

Everything removed from the handbook was added here as far as I can tell.

[0mp]

Thanks!