Features:
+* complement root=, rootflags=, rootfstype= with rootsubdir= which allows
+ mounting a subdir of the root fs as actual root. This can be used as
+ fstype-agnostic version of btrfs' rootflags=subvol=foobar.
+
* add --copy-from and --copy-to command to systemd-dissect which copies stuff
in and out of a disk image