Support stopping and restarting the AHCI command list via toggling
PxCMD.ST from '1' to '0' and back.  This allows the driver a chance to
recover if for instance a timeout occurred due to activity on the
host.

Reviewed by:	grehan
Remove useless debug.

Sponsored by:	The FreeBSD Foundation
Fix build to not bogusly always rebuild vmm.ko.

Rename vmx_assym.s to vmx_assym.h to reflect that file's actual use
and update vmx_support.S's include to match. Add vmx_assym.h to the
SRCS to that it gets properly added to the dependency list. Add
vmx_support.S to SRCS as well, so it gets built and needs fewer
special-case goo. Remove now-redundant special-case goo. Finally,
vmx_genassym.o doesn't need to depend on a hand expanded ${_ILINKS}
explicitly, that's all taken care of by beforedepend.

With these items fixed, we no longer build vmm.ko every single time
through the modules on a KERNFAST build.

Sponsored by: Netflix
Add support for new USB 3G device.

Submitted by:	gabor@zahemszky.hu
MFC after:	1 week
Add basic UEFI boot procedure manpage

Reviewed by:	brueffer
MFC after:	4 days
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D959
Do nothing in vt_upgrade if there is no vt driver

Previously, if no drivers attached at boot we would panic with
"vtbuf_fill_locked begin.tp_row 0 must be < screen height 0".

PR:		192248
Reviewed by:	ray
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D954
* Remove route caching in if_stf.
* Copy necessary in6_ifa on stack instead of playing with refcounts.