ifconfig: make -vht work

Also hide the other vht options on -vht and only show vht40/80/160/80p80
when vht is enabled.

While here fix some whitespace and comments.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian, emaste
Differential Revision: https://reviews.freebsd.org/D48326

(cherry picked from commit 42410c6d682c4e00ce6147f99b51a55f6f3fe075)