Client option secure-auth deployed on all possible layers:
- mysql client command-line and config file option
- mysql_options option MYSQL_SECURE_AUTH
- mysql_real_connect will automatically take into account that option if
  mysql->options.my_cnf_file/my_cnf_group is set


client/client_priv.h:
  added OPT_SECURE_AUTH to enum of all my_read_default_options options.
client/mysql.cc:
  added support for mysql command-line option --secure-auth
include/errmsg.h:
  added return code for option --secure-auth
include/mysql.h:
  added MYSQL_SECURE_AUTH to enum of all mysql_options options.
  added secure_auth flag to MYSQL handle
libmysql/errmsg.c:
  Error messages for option --secure-auth
sql-common/client.c:
  added check for secure-auth in mysql_real_connect:
  if password is provided, and secure-auth is on, then client will
  refuse connecting to pre-4.1.1 server