Move control of create into hook_form_alter() and hook_nodeapi() to prevent numerous problems

Currently, the create op is controlled solely in hook_db_rewrite_sql(). This has advantages and disadvantages. Another method would be to use hook_nodeapi() and hook_form_alter().

This would resolve:
#112307: Cannot create node in taxonomy term without create permission for parent term

and avoid problems like:
#881210: No list or create permissions on any terms may result in incorrect use of default on save
#660668: TAC + revisioning: Empty taxonomy term select list on node/add for regular users

There are a couple of disadvantages. One is that FAPI's #access option can only apply to whole form elements, so if the user has access to create some terms in a vocabulary but not others, we still have to override the whole form element. Another is that any form overrides we provide will by default support only the core taxonomy forms, not other modules that alter those forms (like Content Taxonomy).

For these reason, we might want to make this behavior configurable, and possibly release it only in a new branch since it involves major changes to the way create functions.

Original patch is at:
http://drupal.org/files/issues/tac_881210-9.patch