C++ PATCHes to template parameter parsing

[Jason Merrill]

The first patch is a cleanup hoisted from the c++-concepts branch: on 
the branch we want to be able to parse type/template in multiple places, 
so Andrew factored that out, but it's a good code cleanup on the trunk 
as well.

While looking at this and related code on the branch, I noticed that 
there was a lot of redundant code in cp_parser_template_parameter for 
dealing with parameter packs that we should have handled already in 
cp_parser_parameter_declaration.  I've dealt with that by taking over an 
unused flag in cp_parameter_declarator for passing back that the 
parameter is a pack.  And while I was there I tweaked a permerror that 
wasn't using the permerror function.

Tested x86_64-pc-linux-gnu, applying to trunk.
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