insert_id after succ. mysql_change_user() call.

See also WL 4066.
  
This bug reveals two problems:
  - the problem on the client side which was described originally;
  - the problem in protocol / the server side: connection context
    on client and server should be like after mysql_real_connect()
    and be consistent. The server however just resets character
    set variables to the global defaults.

The fix seems to be as follows:
  - extend the protocol so that the client be able to send
    character set information in COM_CHANGE_USER command;
  - change the server so that it understands client character set
    in the command;
  - change the client:
    - reset character set to the default value (which has been
      read from the configuration);
    - send character set in COM_CHANGE_USER command.