option to use external source for authentication

[bzheng]

I think it would be nice if CACTI can use external source for authentication, such as auth_XXX on Apache, integrated authentication on IIS, or other web form based authentications. What CACTI needs to know is name of the variable that holds the username, which is usually in $HTTP_SERVER_VARS, or $HTTP_SESSION_VARS. And where to re-direct the users to if the varaible is not set. Both can be customized in config.php.
If the user does not exist already in CACTI's internal user table, it should be created with a random password and guest access is allowed.

This way, CACTI does not have to support extra authentication schemas, and existing account repository can be re-used.