Remove #base entirely.

#base has always been the bane of our existence. It allowed us, in the small number of cases where many forms with different IDs all used the same theming, submission, and validation handlers, to map using a base name like my_form rather than the full id, like my_special_form.

This patch eliminates #base entirely, in favor of explicitly setting the #submit, #validate, and #theme handlers. It also simplifies form.inc a bit, and -- this is a biggie -- paves the way for some cleaner solutions to things like 'give each form button its own private callback'.

We want this. It is a happy thing.

Caveats: some of the forms probably don't need the #theme function set, but setting it means that any theme overrides of them won't break.