loop-util: use auto-detect open mode for loop device setup

When callers do not explicitly request read-only mode, pass open_flags
as -1 (auto-detect) instead of hardcoding O_RDWR. This enables the
existing O_RDWR-to-O_RDONLY retry logic in loop_device_make_by_path_at()
which falls back to O_RDONLY when opening the backing device with O_RDWR
fails with EROFS or similar errors.

Previously, callers passed O_RDWR explicitly when read-only mode was not
requested, which bypassed the retry logic entirely. This meant that
inherently read-only block devices (such as CD-ROMs) would fail to open
instead of gracefully falling back to read-only mode.

Also propagate the unresolved open_flags through
loop_device_make_by_path_at() into loop_device_make_internal() instead
of resolving it to O_RDWR early. For loop_device_make_by_path_memory(),
resolve to O_RDWR immediately since memfds are always writable.

In mstack, switch from loop_device_make() to
loop_device_make_by_path_at() with a NULL path, which reopens the
O_PATH file descriptor with the appropriate access mode. This is
necessary because the backing file descriptor is opened with O_PATH,
which prevents loop_device_make_internal() from auto-detecting the
access mode via fcntl(F_GETFL).

src/nspawn/nspawn.c

@@ -6483,7 +6483,7 @@ static int run(int argc, char *argv[]) {
                 if (arg_userns_mode != USER_NAMESPACE_MANAGED) {
                         r = loop_device_make_by_path(
                                         arg_image,
-                                        arg_read_only ? O_RDONLY : O_RDWR,
+                                        arg_read_only ? O_RDONLY : -1,
                                         /* sector_size= */ UINT32_MAX,
                                         FLAGS_SET(dissect_image_flags, DISSECT_IMAGE_NO_PARTITION_TABLE) ? 0 : LO_FLAGS_PARTSCAN,
                                         LOCK_SH,