gpg2 insists on created a trust db even if we tun off all trust db
support. Hence create a temporary home where the trust db is placed, and
remove it after use.
After all it is now much more like strjoin() than strappend(). At the
same time, add support for NULL sentinels, even if they are normally not
necessary.
The old "systemd-import" binary is now an internal tool. We still use it
as asynchronous backend for systemd-importd. Since the import tool might
require some IO and CPU resources (due to qcow2 explosion, and
decompression), and because we might want to run it with more minimal
priviliges we still keep it around as the worker binary to execute as
child process of importd.
machinectl now has verbs for pulling down images, cancelling them and
listing them.
