(bug 36400) API: Fix sorting for iwlinks, langlinks

(bug 36400) API: Fix sorting for iwlinks, langlinks

The iwlinks and langlinks modules continue parameters imply ordering by
page then prefix then title. But in certain modes, the actual queries
use a different ordering, which may result in skipped or repeated
results.

This changeset fixes that. To do so, it needs to re-add an index
iwl_prefix_from_title which was mistakenly removed in 2010 (r69721). And
while it's doing that, it cleans up errors in the sqlite and postgresql
handling of the iwlinks indexes too.

Also, per Asher, make the iwl_prefix_from_title and
iwl_prefix_title_from indexes non-UNIQUE.

Change-Id: I607e8bf9183a2d8152a6127a81c83a0b5bba0c61