COM_CHANGE_USER was always handled like an implicit request to change the
client plugin, so that the client can re-use the same code path for both normal 
login and COM_CHANGE_USER. However this doesn't really work well with old 
clients because they don't understand the request to change a client plugin.

Fixed by implementing a special state in the code (and old client issuing 
COM_CHANGE_USER). In this state the server parses the COM_CHANGE_USER 
package and pushes back the password hash, the user name and the database 
to the input stream in the same order that the native password server side plugin 
expects. As a result it replies with an OK/FAIL just like the old server does thus
making the new server compatible with older clients.

No test case added, since it would requre an old client binary. Tested using 
accounts with and without passwords. Tested with a correct and incorrect 
password.