OMakeroot: move OCAMLPACKS to use for lib and app
About: add description
app: port to Core's Command

Closes #2. However, the semantics were not preserved exactly. For
example, previous API allowed many options to be passed multiple times
as where the new API allows them just once. Anyway, we plan to
re-design the API, so it is not worth assuring an exact match with the
previous behavior.