and ptopgen.

Add link typing to assign. Each virtual link is given a single type.
Each physical link is given one or more types. A virtual link will
only be mapped to a physical link which can satisfy its type. In both
the top and ptop files, the link types are now mandatory, and they
fall at the end of the mandatory link arguments.

This differers from the 'regular' type system in two ways. First, a
plink is not constrained to filling only one type at a time. If we are
using emulated links, a plink could satisfy, say, an 'ethernet' link
and an 'fxp' link at the same time. This seems to more naturally match
the way we'll use link types.  Second, there are no counts assoicated
with link types, as there are for node types. ie. a link is not an
'ethernet:1' link, it's an 'ethernet' link. Presumably, when
multiplexing virtual links onto a physical one, it's bandwidth that's
the factor that limits the multiplexing.

The link type is now tak...