Factor out a libotcore

Factor out a libotcore

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.