Implement dialog auth plugin

[julienschmidt]

MariaDB and Percona Server use the dialog auth plugin instead of MySQL's mysql_cleartext_password plugin for PAM authentication.

This driver should come with built-in support, as PAM authentication is rather common. The implementation complexity seems low. MariaDB's client implementation can be found here: https://github.com/MariaDB/mariadb-connector-c/blob/master/plugins/auth/dialog.c

[buphmin]

I don't know if this will work in every PAM auth setup with mysql/mariadb (we use mariadb). But simply adding a fallthrough in auth.go to mysql_clear_password allowed authentication to work if you follow the rest of the docs on clear passwords. Maybe this will help someone else.

case "dialog":
	fallthrough
case "mysql_clear_password":