[xml2rfc] changes for new ipr documents

[Marshall Rose]

for people who want to try out the new ipr stuff, the web-based service
on http://xml.resource.org has been updated (however, a new .tgz file
has not been released).
    
to recap:
    
1. if the 'ipr' attribute of the <rfc/> element is one of the old values
   ('full2026', 'noDerivativeWorks2026', or 'none'), then there are no
   changes.
    
2. if the 'number' attribute of the <rfc/> element is less than 3708,
   then there are no changes. (this cutoff number may change based on
   what the rfc-editor publishes.)
    
3. otherwise:
    
3a. new values allowed for the 'ipr' attribute:
    
    full3667  =
       IPR Disclosure Acknowledgement

    noModification3667 = 
       IPR Disclosure Acknowledgement
       Derivative Works Limitation (a)

    noDerivatives3667  = 
       IPR Disclosure Acknowledgement
       Derivative Works Limitation (b)
    
3b. for the latter two values, to allow the extraction of a MIB/PIB/etc.,
    use the new 'iprExtract' attribute, which is the anchor of the
    Section containing the thing you're allowing to be extracted.
    
3c. the copyright notice and disclaimer are updated, and a disclaimer of
    validity is added.

/mtr