sd-id128: parse UUID URNs

Accept RFC4122 UUID URN strings with the `urn:uuid:` prefix in
sd_id128_from_string(), while keeping plain 128-bit IDs and regular
UUID strings working as before.

TODO.md

@@ -1262,8 +1262,6 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 - in pid1: include ExecStart= cmdlines (and other Exec*= cmdlines) in polkit
   request, so that policies can match against command lines.
 
-- in sd-id128: also parse UUIDs in RFC4122 URN syntax (i.e. chop off urn:uuid: prefix)
-
 - in sd-stub: optionally add support for a new PE section .keyring or so that
   contains additional certificates to include in the Mok keyring, extending
   what shim might have placed there. why? let's say I use "ukify" to build +