Normalize the received prefixes.

Normalize the received prefixes.

We need to use ldp_applymask() to normalize the received
prefixes. Example: 10.1.1.0/16 -> 10.1.0.0/16.

Additionally, stop using IANA's AF numbers in map->fec.prefix.af and use
AF_INET/AF_INET6 instead. This makes the code much simpler, use AF_IPV[46]
only when necessary (decoding/encoding prefixes).

ok claudio@