Convert he ATTR_ASPATH and ATTR_AS4_PATH handlers in rde_attr_parse()

Convert he ATTR_ASPATH and ATTR_AS4_PATH handlers in rde_attr_parse()
to new ibuf API.

Various aspath functions are modified to work better with ibufs.
aspath_inflate() now only works with ibufs and is a lot simpler.
aspath_verify() does all the checks using the ibuf api and therefor
most length checks can be skipped.
aspath_asprint() and the new internal aspath_strsize() and aspath_snprint()
are totally overhauled -- including some bugs that got squashed.
OK tb@