MDL-9399 auth/ldap: flesh out ntlmsso_* functions

MDL-9399 auth/ldap: flesh out ntlmsso_* functions

Flesh out the functions that get the job done. These will be triggered
by PHP files sitting under auth/ldap/ ...

ntlmsso_magic() - here is where the magic happens. Call it serving
something harmless, like a space GIF, from a URL that is blessed with
Integrated Windows Authentication. If the IWA is successful, it will
set a special session cookie in config_plugins table.

It won't complete the job because when we are under IWA magic we
cannot change (even read!) the session data, as the execution happens
under the OS privileges of the actual user acct being logged in (!!!)

ntlmsso_finish() - if it finds the session set by ntlmsso_magic() and
it hasn't expired, it will finish the job of logging in the user, by
calling authenticate_user_login() and exercising the whole auth
machinery.

user_login() - when called by authenticate_user_login() it will check
to see if there's a session setup by ntlmsso_magic() and OK the login
without really asking the AD backend to proof a password we don't have
anyway.