Add setting to disable IP Router Alert Option

[troglobit]

In 760dd9d support for the IP Option Router Alert was added. Since not all end devices, or even routers, support¹ Router Alert this may lead to loss of function in some setups.

Hence, we need to add support for disabling, or explicitly enabling, this feature.

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¹) They may hard-code IP header lengths to 20 bytes, like mrouted does, on reception.