Add a variable to disable tag wigets on the search and content forms

node_form_alter() currently calls taxonomy_form_all() with $free_tags set to 1. This means that loading search/node on a site with a significant number of tags will likely cause a WSOD due to hitting the PHP memory limit. Since the function is called from a form alter, you can't prevent it from being called via a contrib module. The function was removed entirely in #693362: taxonomy_form_all() is dangerous, but we can't do that for D6. We should instead add a variable check so that site builders can disable listing tags at all.