DTD changes 29 May 17

[Mark Giffin <mark@markgiffin.com>]

I made a few DTD changes based on our discussions in the lwdita SC 
meeting today.

I added the %fn-blocks; entity to provide the trimmed-down content model 
for the <fn> element, per discussion.

I removed the following elements that were not used anywhere. Mostly 
these are related to template-based specialization, which we are not 
working on right now:
<param> (left over from removed <object>)
<specmeta>
<specatt>

We did not discuss this, but I also removed the %spec-atts; entity, 
which was used for specialization attributes and was obviously 
extraneous. This entity defined these attributes: @specmodel, @specrole, 
@importance.

I created a new branch in the repo called spec-dev where all the removed 
specialization stuff still exists if you want to see it.

Mark Giffin