[feature/passwords] Add phpass copyright notice to salted md5 driver

PHPBB3-11610
[feature/passwords] Move old hashing functions to compatibility file

Additionally, older unused functions that have been moved into the password
hashing drivers have been removed from functions.php. Tests have been
adapted to properly work after moving phpbb_hash() and phpbb_check_hash()
into functions_compatibility.php.

PHPBB3-11610