dissect: optionally allow attaching images that do not qualify as DDIs with --attach

It's nice to use --attach since it validates our images. So far, it
strictly insisted on checking that the image was a DDI however, which
limits its use for certain usecases.

Add "--relax" to address this limitation.

shell-completion/bash/systemd-dissect

@@ -37,7 +37,8 @@ _systemd_dissect() {
                      -r --read-only
                      --mkdir
                      --rmdir
-                     --in-memory'
+                     --in-memory
+                     --relax'
         [ARG]='-m --mount -M
                -u --umount -U
                --attach