core: Add DelegateNamespaces=

This delegates one or more namespaces to the service. Concretely,
this setting influences in which order we unshare namespaces. Delegated
namespaces are unshared *after* the user namespace is unshared. Other
namespaces are unshared *before* the user namespace is unshared.

Fixes #35369

src/core/execute.c

@@ -611,6 +611,7 @@ void exec_context_init(ExecContext *c) {
                 .timeout_clean_usec = USEC_INFINITY,
                 .capability_bounding_set = CAP_MASK_UNSET,
                 .restrict_namespaces = NAMESPACE_FLAGS_INITIAL,
+                .delegate_namespaces = NAMESPACE_FLAGS_INITIAL,
                 .log_level_max = -1,
 #if HAVE_SECCOMP
                 .syscall_errno = SECCOMP_ERROR_NUMBER_KILL,