nspawn: make sure --private-users-ownership=no and =off work the same way

We usually want to use "extended booleans" for cases like this, i.e.
that "off", "no" and "0" can be used interchangably for turning
something off.

src/nspawn/nspawn-settings.c

@@ -936,7 +936,8 @@ static const char *const user_namespace_ownership_table[_USER_NAMESPACE_OWNERSHI
         [USER_NAMESPACE_OWNERSHIP_AUTO]  = "auto",
 };
 
-DEFINE_STRING_TABLE_LOOKUP(user_namespace_ownership, UserNamespaceOwnership);
+/* Note: while "yes" maps to "auto" here, we don't really document that, in order to make things clearer and less confusing to users. */
+DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(user_namespace_ownership, UserNamespaceOwnership, USER_NAMESPACE_OWNERSHIP_AUTO);
 
 int config_parse_userns_chown(
                 const char *unit,