Author: Paul Holden
MDL-68712 enrol_self: improve detection/re-use of group enrolment keys.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=cee1103becae2ef766ac4f9aeb793f13f92522cb
http://github.com/moodle/moodle/commit/cee1103becae2ef766ac4f9aeb793f13f92522cb
+ 4.1 en [passwordmatchesgroupkey,enrol_self]