API: Fix up r47214, which strangely had no commit message

API: Fix up r47214, which strangely had no commit message
* Use + instead of array_merge(), the latter messes up keys
* Fix a fatal error ($result not an object)
* Fix an interesting mistake when calling addTables(): addTables('foo', 'bar') doesn't add both tables, but adds foo AS bar
* Fix warning about $row->pt_namespace not being set

Commit message for r47214:
API: Refactor ApiQueryInfo
* Move result generating code from execute() to extractPageInfo()
** Merge code for existing and missing titles
* Don't loop over existing and missing titles separately, but on both in one go
* Move protection query code from execute() to getProtectionInfo()
** Merge code for existing and missing titles here as well
* Move subjectid/talkid query code from execute() to getTSIDs()
* Document some functions