Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove the ah_desc.h reference; it's not needed.

I'm using these descriptor header files in userland and I'm trying to
avoid populating a compatibility ah_desc.h file.

Replace Id keyword with FreeBSD keyword and set the svn props correctly.
No functional change.

add FreeBSD property

import ath hal

virgin import of ath hal