authpdo: fix mybb schema for MySQL 5.7+

https://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html

> In MySQL 5.7.5, these SQL mode changes were made: ... Strict SQL mode for trans

authpdo: fix mybb schema for MySQL 5.7+

https://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html

> In MySQL 5.7.5, these SQL mode changes were made: ... Strict SQL mode for transactional storage engines (`STRICT_TRANS_TABLES`) is now enabled by default.

https://dev.mysql.com/doc/refman/5.7/en/data-type-defaults.html

> For data entry into a NOT NULL column that has no explicit DEFAULT clause, if an INSERT or REPLACE statement includes no value for the column, or an UPDATE statement sets the column to NULL, MySQL handles the column according to the SQL mode in effect at the time:
>
> If strict SQL mode is enabled, an error occurs for transactional tables and the statement is rolled back. For nontransactional tables, an error occurs, but if this happens for the second or subsequent row of a multiple-row statement, the preceding rows will have been inserted.

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AuthPDO: allow in-SQL pasword checking

This is a feature that was present in the old authmysql plugin. It
allows administrators to specify an SQL string that is used for explicit
password checking i

AuthPDO: allow in-SQL pasword checking

This is a feature that was present in the old authmysql plugin. It
allows administrators to specify an SQL string that is used for explicit
password checking instead of letting DokuWiki compare the hashes. This
is useful where application implement their own salt/hashing thing.

This comes with a test case for the MyBB forum software. Dump kindly
provided by William Kitchen

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