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.


client/client_priv.h:
  Declare a function, used in libmysql.c and client.c.
libmysql/libmysql.c:
  1. Reset character set on the client in mysql_change_user().
  2. Send character set to the server in COM_CHANGE_USER command.
mysql-test/t/mysql_client_test.test:
  mysql_client_test.log is used by the test suite.
  
  Use mysql_client_test.out.log to collect mysql_client_test
  real output.
sql/sql_parse.cc:
  Switch character set in COM_CHANGE_USER.
tests/mysql_client_test.c:
  Test case for BUG#30472.