Basically a flavour of gnugpg, which solely containts `bin/gnupg`.

Adding `systemd-importd` to the build, so that `machinectl`s `import-.*`
may actually do anything. Currently they fail with

```
Failed to transfer image: The name org.freedesktop.import1 was not provided by any .service files
```
as `systemd-importd` is not built. Also registers the regarding dbus
api and service in the systemd module.

This adds a test downloading an nspawn container via http, and ensures
sha256sum verification and gpg signature verification work.