support convertedFrom for rfc building

[miekg]

To help us debug your issue please add these details.

Can't build an RFC without warnings:

514.xml(5): Warning: Expected explicit values for year, month, and day, but found None, None and None
3514.xml(3): Warning: Expected a <link> with rel='convertedFrom' providing the datatracker url for the origin draft.

We need a convertedFrom in the toml, maybe 7749 can use it as well.

See examples in the rfc dir

[miekg]

Actually this is <link>: https://tools.ietf.org/html/rfc7991#section-2.30

   o  The current International Standard Serial Number (ISSN) for the
      RFC Series.  The value for the "rel" attribute is "item".  The
      link should use the form "urn:issn:".

   o  The Digital Object Identifier (DOI) for this document.  The value
      for the "rel" attribute is "describedBy".  The link should use the
      form specified in [RFC7669]; this is expected to change in the
      future.

   o  The Internet-Draft that was submitted to the RFC Editor to become
      the published RFC.  The value for the "rel" attribute is
      "convertedFrom".  The link should be to an IETF-controlled web
      site that retains copies of Internet-Drafts.

   o  A representation of the document offered by the document author.
      The value for the "rel" attribute is "alternate".  The link can be
      to a personally run web site.

I think we can just add link = and then figure out from the url/string what rel should be.

[miekg]

probably should drop value from seriesInfo (this may happen in the spec), let's see what comes out of it.

[miekg]

the xml2rfc V3 is still in flux, current stuff works, when (if?) the new RFC comes out we'll revisit, for now we try follow whatever xml2rfc does.