bgpd: Convert to default_af[afi][safi]

bgpd: Convert to default_af[afi][safi]

Introduces bgp->default_af to selectively enable various default
afi/safis to be inherited by new peers.
Makes default_af flag logic consistent for all address-families, i.e.
instead of a "no default" flag for ipv4 and a "default" flag for ipv6,
just use "default" for both and make it true for ipv4 by default.
Removes old BGP_FLAG_NO_DEFAULT_IPV4 and BGP_FLAG_DEFAULT_IPV6, and
cleans up bgp->flags bit definitions to avoid gaps for unused bits.
Signed-off-by: Trey Aspelund <taspelund@nvidia.com>