Author: Amaia Anabitarte
MDL-63068 user: Allow underscore for profile custom fields shortname
http://git.moodle.org/gw?p=moodle.git;a=commit;h=856c51a80b399ee852c34c664cc4e52ed3b25625
http://github.com/moodle/moodle/commit/856c51a80b399ee852c34c664cc4e52ed3b25625
+ 3.6 en [profileshortnameinvalid,core_admin]