MDL-42177 libraries: Additional checking of the fullnamedisplay language string.

If the language string is missing firstname or lastname, then when displaying the
user edit form it defaults to 'firstname lastname'

Previous logic has been moved out to new functions with unit tests.
MDL-42177 libraries: Unit tests for new functions that deal with user name fields.

functions with unit tests: get_required_user_name_fields, get_enabled_user_name_fields,
get_disabled_user_name_fields.