This checks if plugin names are valid and only loads valid plugin files,
this could prevent some errors from wrong upgrades as described in
FS#2464.

The BOM at the beginning of some plugin.info.txt files prevented the
plugin manager from correctly reading the base property and thus it
installed the plugin in the wrong directory.

update the url to the latest one used in the manager (FS#2463)

This fixes the test for existing subscriptions by not only testing if
the subscription for the deepest namespace level is for the current page
but by simply testing all levels. Test case is included, it fails
without this change.

FIX for FS#2463 <https://bugs.dokuwiki.org/index.php?do=details&task_id=2463>

this function was removed in PHP 5.4.0

The class loader isn't available at that place in inc/init.php so the
check if the sitemap is compressed didn't work.

Compressing a gzip file again for transport is standards compliant, but
some clients assume that the file is only compressed once then and don't
remove the outer compression layer. This could disable compression in
too many cases theses cases should be rare and shouldn't cause any
problems.