The plugins check screen now provides buttons to cancel installation of
a plugin. Available only for new installations (not upgrades) and for
additional plugins (not standard), given that the web server process has
write access to the plugin folder.

This has also been reported as MDL-48535.

As a part of the patch, there is improved processing of page URLs during
the upgrade. All this dancing around $reload URL is not needed once the
$PAGE->url is properly set to guide the admin on the correct page during
the upgrade process.