Fully integrate zstd into base system

[Simon Burge <simonb%NetBSD.org@localhost>]

Hi folks,

We currently have zstd in our source tree, but we only install the
zstd binary.

Is there any reason to not fully integrate zstd by installing the
library and include files?  We can then easily hook zstd into zcat
and libarchive.  This will also make it easier to experiment with
using zstd for our sets.

We could try to use the new private library concept added in December
that that doesn't look to easily support sharing between different
parts of the source tree and would also need a way to use private
includes (maybe obj-relative so they are not published somewhere in
/usr/include?).

zstd is also quite stable.  The current 1.5.7 was released about 11
months ago, and previous 1.5.6 was released 11 months before that.

Cheers,
Simon.