derivations: Add #:substitutable?, distinguished from #:local-build?.

Fixes <http://bugs.gnu.org/18747>.

* guix/derivations.scm (substitutable-derivation?): Rewrite to check for
  "allowSubstitutes".
  (derivation): Add #:substitutable? parameter.
  [user+system-env-vars]: Honor it.
  (build-expression->derivation): Add #:substitutable? and honor it.
* guix/gexp.scm (gexp->derivation): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and substitutes,
  non-substitutable build"): Use #:substitutable? instead of #:local-build?.
  ("substitutable-derivation?", "derivation-prerequisites-to-build and
  substitutes, local build"): New tests.
* guix/download.scm (url-fetch): Adjust comment.
* guix/git-download.scm (git-fetch): Likewise.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Use #:substitutable?
  instead of #:local-build?.
* doc/guix.texi (Derivations, G-Expressions): Adjust accordingly.