derivations: Add #:allowed-references 'derivation' parameter.

* guix/derivations.scm (derivation): Add #:allowed-references
  parameter.
  [user+system-env-vars]: Honor it.
* tests/derivations.scm ("derivation #:allowed-references, ok",
  "derivation #:allowed-references, not allowed",
  "derivation #:allowed-references, self allowed",
  "derivation #:allowed-references, self not allowed"): New tests.
* doc/guix.texi (Derivations): Document #:allowed-references.