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

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

The last big commit: let usr.sbin/ use WARNS=6 by default.

Remove superfluous CFLAGS lines. Use the conditional '?=' construct
for WARNS lines.

Submitted by: ru

Bring a working snapshot of hwpmc(4), its associated libraries, userland utilities
and documentation into -CURRENT.

Bump FreeBSD_version.

Reviewed by: alc, jhb (kernel changes)