Convert most attributes in rde_attr_parse() to new ibuf API.

Convert most attributes in rde_attr_parse() to new ibuf API.

This skips ATTR_ASPATH and ATTR_AS4_PATH for now, those will follow soon.
Reshuffle checks a little bit. While ibuf_get does ensure that enough data
is available do a precise size check to ensure that only the expected amount
of data is available.

OK tb@