fix mtu parent setting and add max-payload for pppoe

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translation fix

(legacy) change behaviour of associated nat rules (https://github.com/opnsense/core/issues/695), show rule but hide edit and duplicate buttons

If for some reason a config has "typeless" rules, which don't belong to any active nat rule the user won't notice this anymore (and doesn't have the ability to remove them).
This can happen with imports from old configs for example or manually changed configs.

Closes #726

There are a few cases where the gateway code is eligible
to drop a gateway on grounds of being disabled, inactive,
unconfigured or where the actual interface assigned is
disabled.  This is undesire because users that configure
a gateway won't be able to use it.

There are no bad side effects from the former by selecting
a whatsoever dysfunctional gateway, because the problem still
has to be fixed by the user in the gateway setup itself.

Worst of all, removing a disabled but used gateway makes
it not show up on edit, effectively wiping its setting on
save.  This breaks POLA.

While there, also enable the null routes for IPv4 and IPv6.

PR: https://forum.opnsense.org/index.php?topic=2100

PR: https://forum.opnsense.org/index.php?topic=2012.0

Lookie how far we've come...