Pass block.module queries through db_rewrite_sql()

We regularly get requests for more finely-grained control of block visibility. See e.g. "Block display conditional on content type" http://drupal.org/node/72143 and Block display conditional on taxonomy terms http://drupal.org/node/20382.

For roles we created a new table and explicitly modified the SQL (and UI) in block.module.

A more flexible approach, though, might be to use db_rewrite_sql() and allow other modules to alter block display. Then we could use a combination of hook_form_alter() and hook_db_rewrite_sql() to implement very finely-tuned access control in contrib modules, without needing further changes to block.module.