build - Add zstd to the build (libzstd as private library)

* Bring in contrib/zstd from FreeBSD.

* Add libzstd as a private library (i.e. priv/libprivate_zstd) to
avoid confusion. (Note that Fre

build - Add zstd to the build (libzstd as private library)

* Bring in contrib/zstd from FreeBSD.

* Add libzstd as a private library (i.e. priv/libprivate_zstd) to
avoid confusion. (Note that FreeBSD also has zstd in base as
a private library).

* Add zstd, unzstd, zstdcat, zstdmt utilities.

* Integrate into libarchive, initrd, dsynth, and misc other
base system elements.

Ported-from: FreeBSD

show more ...