bgpd: Improve group overrides for AF filters

bgpd: Improve group overrides for AF filters

This commit adds the same peer-group override capabilites as d122d7cf7
for all filter/map options that can be enabled/disabled on each
address-family of a BGP peer.

All currently existing filter/map options are being supported:
filter-list, distribute-list, prefix-list, route-map and unsuppress-map

To implement this behavior, a new peer attribute 'filter_override' has
been added together with various PEER_FT_ (filter type) constants for
tracking the state of each filter in the same way as it is being done
with 'af_flags_override'.

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