zstd/Makefile: Nix trailing whitespace.

No functional change intended.

zstd, libuv: Fix conditional LIBISPRIVATE=pic setting.

This was wrong for arbitrary MKPIC=no builds, and is unnecessary for
sun2 builds now that bsd.lib.mk already handles this case
appropriately.

try to fix the sun2 build: don't build pic since we are linking statically.

fix sun2 build.

make zstd a private library (for now)

list zstd.1 in man pages

Hook zstd to the build and enable it for libarchive and file.

fix names.

build libzstd.pc

Add build glue.

Import zstd-1.5.6 from:
https://github.com/facebook/zstd/releases/download/v1.5.6/zstd-1.5.6.tar.gz