Fix dissection of RFC 4684 routing information.

Fix dissection of RFC 4684 routing information.

RFC 4684, sadly, doesn't do a very good job of describing what a "route
target" is.  In practice, it appears to be a Route Target extended
community, as per RFC 4360 section 4 "Route Target Community".  Those do
*not* have the same format as an RFC 4364 Route Descriptor.

So just take the code to dissect an extended community, put it in a
subroutine, change it to directly print information rather than filling
in a static string buffer (C support for strings is spectacularly bad -
large variable-length objects are hard, let's go shopping!), and call
that routine when dissecting RFC 4684 routing information.