parse-util: add explicit parsers for MTU values

parse-util: add explicit parsers for MTU values

We use MTUs all over the place, let's add a unified, strict parser for
it, that takes MTU ranges into account.

We already have parse_ifindex() close-by, hence this appears to be a
natural addition, in particular as the range checking is not entirely
trivial to do, as it depends on the protocol used.