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.
MDL-36669 Filters: Remove the last ref to the UFO library.
Merge branch 'm26_MDL-36669_UFO_references_in_filelib' of https://github.com/scara/moodle into MOODLE_26_STABLE
Merge branch 'wip_MDL-41593_m26_preventexecdefault' of https://github.com/skodak/moodle into MOODLE_26_STABLE