Implement htprotmode handling.

This is separate to 11g protection - the default is to RTS protect
11n frames, including A-MPDU frames.

Tested:

* Intel 5100, STA mode


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271249 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f