I've left stubs in place to allow setting the JDK in use,
but I have not yet implemented that, so this only adds support
for `programs.jdk.installed`

I thought I understood bash quoting. I do not. Here's an example:

```
[sam](~)$ for jdk in "/nix/store/fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402/*.jdk"; do echo $jdk; echo ${jdk//\//_}; done
/nix/store/fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402/zulu-8.jdk
_nix_store_fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402_*.jdk
[sam](~)$ for jdk in /nix/store/fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402/*.jdk; do echo $jdk; echo ${jdk//\//_}; done
/nix/store/fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402/zulu-8.jdk
_nix_store_fq62102p6snl4mix9w0wpzw9a91ck4l1-zulu-ca-jdk-8.0.402_zulu-8.jdk
```