clingo: init at 5.2.2

Note that clasp (included in clingo) is already packaged separately, but
only an earlier version. As it is used by OPAM, but will stop being used
by OPAM later (and I want to grab the name for Clasp the Common Lisp
implementation), I decided to package clingo as a whole (as recommended),
but to leave clasp until OPAM stops needing it.