check_mysql: ignore authentication failure

This patch allows checking if MySQL server is running without providing valid
username and password. Similar to check_ssh plugin it returns MySQL server
version string and protocol number.

Example:
check_mysql -n -H aaa.bbb.ccc.ddd
MySQL OK - Version: 5.0.51a-24+lenny5 (protocol 10)

This is useful for monitoring servers where one does not have administrator
privileges or does not want to grant any privileges for the monitoring station.

To enable this functionality new option --ignore-auth (-n) is added to
check_mysql plugin.

Thanks to Julius Kriukas

Closes #1020
Closes #1178