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

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

Bump date as modified the man page

Submitted by: gjb

ehci tunables are only available when kernel is compiled with USB_DEBUG

PR: docs/163646
Reported by: Momchil Ivanov <momchil@xaxo.eu>
Submitted by: Benjamin Kaduk <kaduk@mit.edu>
Approved by: cperciva
MFC after: 1 week

- Add missing xhci(4) manual page.
- Minor update in some USB manual pages.

MFC after: 3 days
Approved by: thompsa (mentor)

The NetBSD Foundation has granted permission to remove clause 3 and 4 from
their software.

Obtained from: NetBSD

Interrupt transfers and split transactions work now, so remove the
part of the BUGS section that says they don't. Leave the "quite
buggy" comment for now though, as it's still partially true.

Add a manual page for the USB Enhanced Host Controller Driver.

Submitted by: ticso (who obtained the original version from NetBSD)