Allow handling RAs when forwarding is enabled

...to conform with Linux's `accept_ra` sysctl option.

```
  accept_ra - INTEGER
    Accept Router Advertisements; autoconfigure using them.

    It also determines whether or not to transmit Router
    Solicitations. If and only if the functional setting is to
    accept Router Advertisements, Router Solicitations will be
    transmitted.

    Possible values are:
      0 Do not accept Router Advertisements.
      1 Accept Router Advertisements if forwarding is disabled.
      2 Overrule forwarding behaviour. Accept Router Advertisements
        even if forwarding is enabled.

    Functional default: enabled if local forwarding is disabled.
                        disabled if local forwarding is enabled.
```

PiperOrigin-RevId: 372214640

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