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.
I actually wondered the same :)

However, at the moment default_home_page we specify as being 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.