BGP: Disallow AS Sets by default

BGP: Disallow AS Sets by default

For a long time, AS Sets have been considered obsolete
but they were still valid by the original RFC. Recently,
RFC 9774 flipped this and AS Sets are now formally deprecated.

Therefore, all BGP sessions will now by default reject routes
containing AS Sets in their AS Paths. If you want to keep the
old behavior, you may simply state

    allow as set;

in every BGP protocol configuration where you want to accept AS Sets.

This closes #248.