Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r240520:
s/teh/the/g

Approved by: cperciva (implicit)

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Make "options XENHVM" compile for i386, not just amd64 -- a largely
mechanical change. This opens the door for using PV device drivers
under Xen HVM on i386, as well as more general harmonisation of i386
and amd64 Xen support in FreeBSD.

Reviewed by: cperciva
MFC after: 3 weeks

fix UP compilation

- bump __FreeBSD version to reflect added buf_ring, memory barriers,
and ifnet functions

- add memory barriers to <machine/atomic.h>
- update drivers to only conditionally define their own

- add lockless producer / consumer ring buffer
- remove ring buffer implementation from cxgb and update its callers

- add if_transmit(struct ifnet *ifp, struct mbuf *m) to ifnet to
allow drivers to efficiently manage multiple hardware queues
(i.e. not serialize all packets through one ifq)
- expose if_qflush to allow drivers to flush any driver managed queues

This work was supported by Bitgravity Inc. and Chelsio Inc.

Fix IPI support

Header cleanups and addition of IPI declarations for xen

Compile fixes for xen build.

MFC after: 1 month.

Import xen sub-arch includes.

MFC after: 2 weeks