Accept OPAM_foo=1 to tell OPAM that `foo` is true or OPAM_foo=0 that is false.

(OPAM_foo=true and OPAM_foo=false are still valid)
Add new package variables.

Fix #674

I've added:

    opam config var core:version
    opam config var core:pinned
    opam config var core:bin
    opam config var core:lib
    opam config var core:doc
    opam config var core:share

and it's easy to add new ones
Improve error reporting for variables

(eg. `opam config var foo` gives now more informative messages if foo is not defined)
Fix OpamMisc.split when using special chars (as '+').

This is fixing `opam config foo+bar:installed`