#23745: handle duplicate MIME parameter names in new parser.

This mimics get_param's error handling for the most part.  It is slightly
better in some regards as get_param can produce some really weird results for
duplicate *0* parts.  It departs from get_param slightly in that if we have a
mix of non-extended and extended pieces for the same parameter name, the new
parser assumes they were all supposed to be extended and concatenates all the
values, whereas get_param always picks the non-extended parameter value.  All
of this error recovery is pretty much arbitrary decisions...