Remove fork of rpc-light, start using opam for dependencies

After working on this with @jonludlam, the general approach is:
  1. every component which wants to use packages (xapi, rrdd, xenopsd etc) should use opam to install as many libraries as possible into a temporary .opam directory, from the opam repository cache. See the xapi-libs.spec.in in this commit.
  2. components which build libraries (such as xen-api-libs) will still use "ocamlfind install" with the DESTDIR set to the RPM staging path, as before. The binary RPM will therefore contain ocaml libraries in the standard system path (because the install is not done with opam)
  3. components which consume non-opam libraries will "opam install" as much as possible. Libraries installed in the system from an RPM might need to be explicitly copied into the temporary opam system root since there appears to be a problem with the opam ocamlfind's search path.