dissect-image: add flag for explicitly enabling userspace verity signature checking

let's make userspace verity signature checking optional. This adds a
dissection flag to enable the logic and patches through all our users to
enable it by default, thus effectively not changing anything from the
status quo ante. However, know we have a knob to turn this off in
certain scenarios.

src/sysusers/sysusers.c

@@ -2247,7 +2247,8 @@ static int run(int argc, char *argv[]) {
                                 DISSECT_IMAGE_VALIDATE_OS |
                                 DISSECT_IMAGE_RELAX_VAR_CHECK |
                                 DISSECT_IMAGE_FSCK |
-                                DISSECT_IMAGE_GROWFS,
+                                DISSECT_IMAGE_GROWFS |
+                                DISSECT_IMAGE_ALLOW_USERSPACE_VERITY,
                                 &mounted_dir,
                                 /* ret_dir_fd= */ NULL,
                                 &loop_device);