Ensure a random password is set for the root user

The approach is that the script "mysql_install_db" (Perl variant)
supports a new option "--random-passwords".
If this option is set, the script will (after setting the
predefined accounts in the table "mysql.user") run commands
from "mysql_security_commands.sql" (new file).

These commands will set a random password for the root accounts
and set the "password expired" flag,
and they will remove the anonymous accounts.