bgpd: Finalize group-overrides for peer/AF attrs

bgpd: Finalize group-overrides for peer/AF attrs

This commit finalizes the previous commits which introduced a generic
approach for making all BGP peer and address-family attributes
overrideable by keeping track of the configuration origin in separate
internal structures.

First of all, the test suite was greatly extended to also check the
internal data structures of peer/AF attributes, so that inheritance for
internal values like 'peer->weight' is also being checked in all cases.

This revealed some smaller issues in the implementation, which were also
fixed in this commit. The test suite now fully passes and covers all the
usual situations that should normally occur.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>