Description
-----------

* This worklog extends the SQL commands as following.

ALTER USER [IF EXISTS] user [auth_option] [password_option]

    auth_option: {
        IDENTIFIED BY 'auth_string'  [REPLACE 'current_auth_string']
        IDENTIFIED WITH 'auth_plugin' BY 'auth_string'
            [REPLACE 'current_auth_string']
    }

    password_option : {
        PASSWORD REQUIRE CURRENT [DEFAULT | OPTIONAL]
    }

CREATE USER [IF NOT EXISTS] user [auth_option] [password_option]

    password_option: {
        PASSWORD REQUIRE CURRENT [DEFAULT | OPTIONAL]
    }

SET PASSWORD [FOR user] = password_option

    password_option : {
        'auth_string' [REPLACE 'auth_string']
    }

* It adds a new column and a global server variable named
  'password_require_current'.

* It either enforces or exempts the user from asking the current password as
  following :

    -- Column value ------- Global variable value -----------Policy
      --    N         -------       OFF             ----------- exempt
      --    N         -------       ON              ----------- exempt
      --    Y         -------       OFF             ----------- require
      --    Y         -------       ON              ----------- require
      --    NULL      -------       OFF             ----------- exempt
      --    NULL      -------       ON              ----------- require

 * A flag 'uses_replace_clause' is added in the LEX_USER structure which is set
   then the password clause is specified.

 * An instance of enum class Current_password is added in the LEX_ALTER
   structure which is initialized corresponding to PASSWORD REQUIRE CURRENT
   statement specified for the user.

 * A new method 'validate_password_require_current()' is added that's where
   current password specified in the REPLACE clause is validated. Once current
   password is validated its in-memory copy is erases. This method is called
   from set_and_validate_user_attributes() method.

 * <current password> is changed to <secret> in the SET PASSWORD clause in the
   general, slow query log and audit logs

 * <current password> is changed to <secret> in the ALTER USER clause in the
   general, slow query log and audit logs

Review:
-------
RB#19346