Fix handling of failed authentication loading

In the case of a failed authentication initialization, the
authentication setup was simply continued with an unset $auth object.
This restores the previous behavior (before merging #141) of simply
returning after unsetting $auth. Furthermore this re-introduces the
check if $auth is set before checking $auth and removes a useless
check if $auth is true (could never be false).
Revert the search depth behavior changes from #154

This reverts parts of the changes from #154: Before merging the pull
request, a depth of 1 returned just the pages in the root namespace.
With the changes in the pull request, a depth of 1 also returned pages
in subnamespaces of the root namespace (as it was also tested in the test case).
This reverts this part of the changes and a depth of 1 returns just the
pages in the root namespace again.
Fix remaining missing $INPUT uses FS#2577

This adds $INPUT in all places where it was still missing and available.
$INPUT is now also used in places where using $_REQUEST/... was okay in
order to make the code consistent.