Allow printf-style arguments in pager_query

Currently pager_query() is the black sheep of the database query family, because it does not allow for printf-style arguments to be inserted in the query. This is a problem because it introduces developer confusion when moving from an unpaged query to a paged one, and it encourages substitution of variables directly into the query, which can bypass our check_query() security feature.

The attached patch adds this ability to pager_query(). The change is backwards-compatible, but a couple calls to the function in core have been changed to use the new capability.