Do not create shared taxonomy terms.

A "shared" term occurs when two entries in the `wp_term_taxonomy` table share a
single `term_id`, and thereby correspond to the same row in `wp_terms`. This
changeset stops the practice of creating shared terms: each new row in
`wp_term_taxonomy` will receive its own row in `wp_terms`. The new strategy
for term creation depends on whether the installation's database schema is up
to date for 4.1:

* If so, terms are allowed to be created with the same slug as an existing term, as long as they are in different taxonomies and do not share a parent. Thus, a new tag with the slug 'wordpress' can exist alongside a category with the slug 'wordpress'.
* If not, new terms will be forced to have unique slugs. Thus, on an installation containing a category with the slug 'wordpress', a new tag 'WordPress' will get the slug 'wordpress-2'.

Fixes #21950. See #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30240 602fd350-edb4-49c9-b593-d223f7449a82