bgpd: Fix AF-attribute overrides when binding peer

bgpd: Fix AF-attribute overrides when binding peer

The current implementation of the overrides for peer address-family
attributes suffered a bug, which caused all peer-specific attributes to
be lost when the peer was added to a peer-group which already had that
specific address-family active.

This commit extends the *peer_group2peer_config_copy_af* function to
respect overridden flags properly. Additionally, the arguments of the
macros *PEER_ATTR_INHERIT* and *PEER_STR_ATTR_INHERIT* have been
reordered to be more consistent and easy to read.

This commit also adds further test cases to the BGP peer attributes test
suite, so that this kind of error is being caught in future commits. The
missing AF-attribute *distribute-list* has also been added to the test
suite.

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