MEDIUM: checks: Make post-41 the default mode for mysql checks

MEDIUM: checks: Make post-41 the default mode for mysql checks

MySQL 4.1 is old enough to be the default mode for mysql checks. So now, once a
username is defined, post-41 mode is automatically used. To do mysql checks on
previous MySQL version, the argument "pre-41" must be used.

Note, it is a compatibility breakage for everyone using an antique and
unsupported MySQL version.