openjdk: Ugly hack to fix nix-env installation

Nix-env always wants to install all outputs of a derivation, but this
failed for openjdk/openjre because openjdk has some symlinks (such as
bin/java) to openjre.  Also, it prevents installing the openjre
without the openjdk.  So as a workaround, mess with the "outputs"
attribute to fool nix-env.

Also, give openjre a separate name and description.

Fixes #1535.