Author: Amaia Anabitarte
MDL-63068 user: Allow underscore for profile custom fields shortname
http://git.moodle.org/gw?p=moodle.git;a=commit;h=bcd99539b728f15f16184edf30b3541c9ee995f5
http://github.com/moodle/moodle/commit/bcd99539b728f15f16184edf30b3541c9ee995f5
+ 3.4 en [profileshortnameinvalid,core_admin]