dissect: make using a generic partition as root partition optional

In preparation for reusing the image dissector in the GPT auto-discovery
logic, only optionally fail the dissection when we can't identify a root
partition.

In the GPT auto-discovery we are completely fine with any kind of root,
given that we run when it is already mounted and all we do is find some
additional auxiliary partitions on the same disk.

src/test/test-dissect-image.c

@@ -43,7 +43,7 @@ int main(int argc, char *argv[]) {
                 return EXIT_FAILURE;
         }
 
-        r = dissect_image(d->fd, NULL, 0, 0, &m);
+        r = dissect_image(d->fd, NULL, 0, DISSECT_IMAGE_REQUIRE_ROOT, &m);
         if (r < 0) {
                 log_error_errno(r, "Failed to dissect image: %m");
                 return EXIT_FAILURE;