Copy stable/11@r303970 to releng/11.0 as part of the 11.0-RELEASE
cycle.

Prune svn:mergeinfo from the new branch, and rename it to RC1.

Update __FreeBSD_version.

Use the quarterly branch for the default FreeBSD.conf pkg(8) repo and
the dvd1.iso packages population.

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

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Merge xz 5.2.0.

This brings support for multi-threaded compression. This brings close
N times faster compression where N is the number of CPU cores.
Because of this, liblzma now depends on libthr.

Soon libarchive will be modified to use the new lzma API.

Thanks to antoine@ for the exp-run.

Differential Revision: https://reviews.freebsd.org/D1786
Reviewed by: bapt

Vendor import of xz-5.2.0 (stripped)