Support for authentication plugins

[dveeden]

Besides the default mysql_native_password there are more plugins available:

• mysql_old_password (pre 4.1 passwords, support for it will be dropped in MySQL 5.7)
• sha256_password (since 5.6?, more secure and stored a salted sha256 hash in authentication_string (instead of storing a non salted SHA1 hash in the password field)
• unix socket authentication
• PAM authentication
• Windows authentication
• No login (similar to /usr/sbin/nologin)

This PR makes it possible to define users which are authenticated by an authentication plugin.

Possible improvements:

• Support CREATE USER xxx IDENTIFIED WITH yyy USING zzz
• It seems like MariaDB 10.x+ uses IDENTIFIED VIA instead if IDENTIFIED WITH
• There is no good way to change the authentication plugin (See https://bugs.mysql.com/bug.php?id=67449 for details)
• For sha256_password old_password=2 must be set if you call PASSWORD()
• The authentication plugin is not loaded automatically

See also:
https://dev.mysql.com/doc/refman/5.7/en/authentication-plugins-available.html
https://mariadb.com/kb/en/mariadb/documentation/plugins/unix_socket-authentication-plugin/
https://mariadb.com/kb/en/mariadb/documentation/plugins/pam-authentication-plugin/

[hunner]

Thanks! Could you squash this into a single commit, or multiple if that would be more apt?

[dveeden]

I've squashed everything into one commit