Support for updated specification

[dret]

RFC 4918 registers all the headers of RFC 2518 (which it updates), except for one (Status-URI, just added here: a58dff3). currently, there is no way to properly represent this dependency, so that one spec can say that it updates another, and then overwrites whatever also has been specified in the updated spec.
This might get as complicated as distinguishing between "obsoleting" and "updating", so fixing this issue probably is not going to be trivial.

[dret]

another example: LDP updates "describedby" (originally from POWDER) at http://www.w3.org/TR/ldp/#link-relation-describedby

[dret]

added LDP "describedby" at 0e8e6f2 (currently this results in a duplicate entry in the linkrel list)

[dret]

not perfect, but at least grouping header fields with the same name in one table row: 5e4b0a9

[dret]

generally speaking, these kinds of dependencies maybe should better be described externally. at least until there's a clear requirement for the kinds of dependencies that need to be describable.