dissect-image: rename verity flag booleans

Let's make the booleans indicating verity state a bit more descriptive.

Let's rename:

    can_verity → has_verity: because that's really what this about
    whether verity data is included in the image. Whether we actually
    can use it is a different story.

    verity → verity_ready: this one should tell us if we have everything
    need to actually set it up, hence explicitly say "ready to use" in
    the name.

No change in behaviour. Just a bit of renaming.

src/nspawn/nspawn.c

@@ -5710,7 +5710,7 @@ static int run(int argc, char *argv[]) {
                 if (r < 0)
                         goto finish;
 
-                if (!arg_verity_settings.root_hash && dissected_image->can_verity)
+                if (!arg_verity_settings.root_hash && dissected_image->has_verity)
                         log_notice("Note: image %s contains verity information, but no root hash specified! Proceeding without integrity checking.", arg_image);
 
                 r = dissected_image_decrypt_interactively(