API: Fix ApiQueryBacklinks logic and use *_from_namespace
The original intent of this patch was to have ApiQueryBacklinks use the
*_from_namespace fields added in Icca99b6a. It does that, but in the
process several other bugs were found in the module:
* Continuation could skip pages when blredirect was used.
* The result object would be populated incorrectly if
$wgAPIMaxResultSize was hit and blredirect was used.
* Continuation could (probably) skip or (maybe) repeat pages when
blredirect was used and $wgAPIMaxResultSize was hit.
In the process of analyzing and fixing these problems, the code was
heavily refactored.
Change-Id: I32381c0f082d2f8e063af99ee353ae003c163c23