Merge remote-tracking branch 'upstream/8.x' into kernel

Resolve conflicts in cron improvements.

Conflicts:
	core/INSTALL.txt
	core/includes/bootstrap.inc
	core/modules/simpletest/drupal_web_test_case.php
	core/modules/system/system.install
	core/modules/system/system.module
	core/modules/system/system.test
	core/scripts/cron-curl.sh
	core/scripts/cron-lynx.sh