core: Fix SQL defaults for filtered backlog fetch

Modify the SQL queries for *_filtered.sql backlog fetching to allow
specifying an invalid type of "0" or "-1" to match any message type.
This allows filtering backlog fetching by flag without restricting by
type.

Also modify the "= 0" check for :flag to "<= 0".  This matches the
function definitions and fixes specifying "-1" instead of "0".
Without this change, specifying "-1" will result in missing messages!

Committed separately from the new forward backlog fetching in case
issues are found, making reverting easier.