The great form.inc cleanup tour, part 1

This patch cleans up various parts of form.inc. It does not change any functionality.

This patch and a few more similar ones are long overdue, form.inc has too much code that can be simplified thanks to newer FAPI techniques.

Here's what is changed by the patch:

• Set $form_state['submitted'] = TRUE for programmed forms in drupal_form_submit(), not in drupal_process_form()
• Introduce a new $form_state['process_input'] flag that is set to TRUE if the form has input and the form ID from $_POST matches or if the form is programmed. This check was performed many times seperately before. Now it's much cleaner.
• Better variable names: $form is now $element in form_builder et. al. form_builder processes elements, after all. Also renamed $form_item to $element in some functions. Consistency and logical names ++.
• Renamed some other variables and added comments here and there. Updated some comments that stayed the same since 2006 or something while the code around 'em changed. Will do more of those in a follow up.
• Made several if() checks simpler. We have default values for $form_state, so no need to do an isset() each time.
• Removed all static vars from form_builder. They are ugly. They rely on global state. They have to go. That's what $form_state is for.
• We used to pass a $complete_form variable containing a complete form to various process and validate functions. We needed static vars for this. So instead, now we set a $form_state['complete form'] property that contains a copy of the complete form.

As I said, no functionality changes, just cleanup. There's more stuff like this in form.inc that should be done, but this is a start and keeps the patch size reviewable. Patches like this or upcoming ones in the same spirit should also finally make it a bit easier for new developers to understand form.inc by simplifying the code flow. Would be great to get this in soon. I have more of those in the works..