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

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

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

Approved by: re (implicit)

Migrate the Xen hypervisor clock reading routines into something
sharable.

Say hello to a very basic, read-only, Xen Hypervisor RTC.

The hypervisor doesn't provide a single "TOD" - it instead provides a
"start time" and a "running time". These are added together to form
the current TOD. The TOD is in UTC.

This RTC is only (initially) designed to be read at startup. There's
some further poking that needs to happen to pick up hypervisor time
changes (ie, by the Dom0 time being adjusted by something). This
time adjustment currently can cause "weird stuff" in the DomU clock;
I'll begin investigating and repairing that in subsequent commits.

PR: 135008