RFC6472 discourages the use of AS_SET segements in ASPATH attributes.

RFC6472 discourages the use of AS_SET segements in ASPATH attributes.
The main reason is that AS_SET does not play nice with RPKI ROA.

Introduce a per neighbor and global config option
    'reject as-set yes' and 'reject as-set no'
If set to yes received UPDATES with AS_SET segements are rejected.
This is done the same way other ASPATH soft-errors are handled. The UPDATE
is marked invalid and all prefixes are treated as withdraws.
`bgpctl show rib in error` can be used to show prefixes that where denied
and treated as withdraws because of errors.

By default this feature is off.

OK benno@