MDL-41593 admin: use default setting if preventexecpath active

This resolves problems when installing Moodle
and addons when preventexecpath active.

Please note the admin_setting_configfile is still
incorrectly restricted - this will be resolved by
MDL-40949.
Merge branch 'wip_MDL-41593_m28_preventexecdefault' of https://github.com/skodak/moodle
MDL-47736 editor_atto: exclude <br> from being restored
Merge branch 'MDL-47736_master_fix' of git://github.com/markn86/moodle