Convert make_IPAddressFamily to CBS/CBB

Convert make_IPAddressFamily to CBS/CBB

The IPAddrBlocks type, which represents the IPAddrBlocks extension,
should have exactly one IPAddressFamily per AFI+SAFI combination to
be delegated. make_IPAddressFamily() first builds up a search key
from the afi and safi arguments and then looks for an existing
IPAddressFamily with that key in the IPAddrBlocks that was passed
in. It returns that if it finds it or allocates and adds a new one.

This diff preserves the current behavior that the afi and *safi
arguments are truncated to 2 and 1 bytes, respectively. This may
change in the future.

ok inoguchi jsing