Experiment with also reading query data from multipart

If any of the form items are files, add then to a
tiddlyweb.input_files environ variable so they could be read
later. We do _not_ want to read them automatically.

May kill this enitrely, but trying it because if you want to
csrf while also allowing file uploads over post, this is needed
(or big changes elsewhere).
Store the entire fieldstorage item as we need all the data
extract methods on post handling

further cleanups coming, and some docs
Add docs to the extracted query processing methods.