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

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

Use the newly brought %U macro.

Add references to scc(4) as it hangs off of ebus(4) and sbus(4)
respectively.

Document a lot of sparc64 internals.

Submitted by: marius
Reviewed by: ru
Approved by: re (blanket)
X-MFC: not before the code in RELENG_5 matches
these man pages or vice versa