[ticket/12352] Add passwords driver for passwords that should be converted

This driver will only be used for getting the new $CP$ prefix that will signal
that the hash is a legacy hash that needs to be converted.

PHPBB3-12352
[ticket/12352] Use $CP$ prefix for converting passwords in manager

PHPBB3-12352
[ticket/12352] Add $CP$ prefix to passwords that need to be converted

Also set user_pass_convert to 0 in the process of doing so.

PHPBB3-12352