A separate fix for 5.1 (as 5.1 and 5.5 have seriously
differged in the related pieces of the code).
A patch for 5.5 was approved earlier.

Problem: ucs2 was correctly disallowed in "SET NAMES" only,
while mysql_real_connect() and mysql_change_user() still allowed
to use ucs2, which made server crash.

Fix: disallow ucs2 in mysql_real_connect() and mysql_change_user().

  @ sql/sql_priv.h
    - changing return type for thd_init_client_charset() to bool,
      to return errors to the caller

  @ sql/sql_var.cc
    - using new function

  @ sql/sql_connect.cc
    - thd_client_charset_init:
      in case of unsupported client character set send error and return true;
      in case of success return false
    - check_connection:
      Return error if character set initialization failed

  @ sql/sql_parse.cc
    - check charset in the very beginnig of the CMD_CHANGE_USER handling code

  @ tests/mysql_client_test.c
    - adding tests