Add requirements for not advertising default router/route on AIL

[EskoDijk]

Based on the original comment: https://mailarchive.ietf.org/arch/msg/snac/SXIt8RGKYyqDsg41Kucg-qVhIj4/ and the response: https://mailarchive.ietf.org/arch/msg/snac/ZXCDdBUjFn39Vr6sc5lZiEqo4NE/

it is proposed to add requirements for the stub router behavior on the AIL regarding:

1. advertising itself as a default router on the AIL;
2. advertising a default route on the AIL.

For correct operation the stub router MUST NOT advertise both of these. While Section 5.4 has requirements for default router/route advertising on the stub network, there is no section yet with specific requirements for the AIL side. This needs to be added.

(FYI @auerswal)

[EskoDijk]

See also some more detailed suggestions at the bottom of mail: https://mailarchive.ietf.org/arch/msg/snac/cST1bdMJ7g52nnws0Si71jAF7io/

[EskoDijk]

This issues was self-assigned to @EskoDijk