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'.

Additional comments have also been included to make it easier to understand the logic.
MDL-42173 administration: Fix alternate name warnings during registration
MDL-42173 administration: Fix alternate name warnings generated when a signup form is submitted