There's no reason to have these distinct really.  If we're using
libsodium, we want it in the same places we're using openssl.

Prep for further refactoring.

This will contain logic shared between ostree-prepare-root
and libostree-1.so.  It will just link to libgio.so, so as
to avoid pulling in e.g. libcurl and other things.

In other words, `ostree-prepare-root` will not link to `libostree-1.so`,
but will pull in just what it needs from this library.

If requested, by specifying ot-composefs=signed=/path/to/pub.key then
the commit object is validated against the specified ed25519 public
key, and if valid, the composefs digest from the commit object is used
to ensure we boot the right digest.