derivations: Add #:disallowed-references.

* guix/derivations.scm (derivation): Add #:disallowed-references.
[user+system-env-vars]: Honor it.
(build-expression->derivation): Likewise.
* tests/derivations.scm ("derivation #:disallowed-references, ok")
("derivation #:disallowed-references, not ok"): New tests.
* doc/guix.texi (Derivations): Adjust accordingly.