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

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

Update contrib/xz-embedded to build with new GEOM_UNCOMPRESS module.

Approved by: adrian (mentor)

Initial copy of xz-embedded to sys/contrib/.

The upcoming geom_compress module (a read-only gzip/ulzma translation layer,
similar to what geom_uzip does) will leverage parts of this.

Import xz-embedded from git.

This is from commit hash '48f4588342f4a4e0182a6740e25675fd8e6c6295'.