Use config_get_global for global configuration values

This provides optimal performance, as well as ensuring that there is
no chance of a user managing to find a way to get one of the 'secure'
configuration options set in the database level.
And as per other comment:

"At the moment allow_anonymous_login is specified as a global only variable.

This commit is ensuring we call config_get_global for global only variables for performance, and not about changing whether any variables are considered global.

I'd consider a patch to change a global configuration variable as a separate change to this.
"

In terms of your comment about checking if a tool supports etc, I'm not sure I actually understand what you are saying :)