Added check for display_admin_url config item which if not 
set to 1 will prevent the server from revealing the admin URL
when the user visits the incorrect domain
(more useful in combination with the musthost config item)

The default value for this is 1 (to preserve backward
compatibility).

I've inverted the comparison of display_admin_url so 
that it must be set to 0 in order to disable it

changed display_admin_url to hide_admin_url and 
changed comparator to test if this value is not set to 1