Author: Amaia Anabitarte
MDL-63068 user: Allow underscore for profile custom fields shortname
http://git.moodle.org/gw?p=moodle.git;a=commit;h=de103e23293e5db9c231d132509ebe13001a7f9e
http://github.com/moodle/moodle/commit/de103e23293e5db9c231d132509ebe13001a7f9e
+ 3.5 en [profileshortnameinvalid,core_admin]