WL#1976: Move opt_secure_auth to the global system variables

Currently in check_user(), we take this mutex:
>   my_bool opt_secure_auth_local;
>   pthread_mutex_lock(&LOCK_global_system_variables);
>   opt_secure_auth_local= opt_secure_auth;
>   pthread_mutex_unlock(&LOCK_global_system_variables);
It would be simpler and faster if we moved opt_secure_auth to
global_system_variables, and make opt_secure_auth_local a read-only session
variable, because then opt_secure_auth would automatically copied to
opt_secure_auth, under mutex, by THD::init():
  pthread_mutex_lock(&LOCK_global_system_variables);
  variables= global_system_variables;
  
  pthread_mutex_unlock(&LOCK_global_system_variables);
and then we can remove the copy done in check_user(). We save one mutex
lock/unlock per connection at the cost of having the opt_secure_auth_local
stored in THD.
As a consequence, we would need in fact to do this copy (with mutex) when CHANGE
USER.

This is a suggestion coming from a discussion between Konstantin and Guilhem
Date: Thu, 8 Jul 2004 00:23:40 +0400
From: Konstantin Osipov 
To: Guilhem Bichot 
Cc: dev-public@mysql.com
Subject: Re: Why do we need LOCK_global_system_variables when testing
 opt_secure_auth once per connection ?