What is this type of screw head?
Hard to tell from the picture, but based on the description, that sounds like a one-way screw. See this page for some more information. Basically, the design is to make it easy to install with a regular screwdriver, but hard to remove, which is what you want on a lock. To remove it, you may need to use a screw extractor - basically you drill a hole into the screw, grab it with an extractor, and then unscrew the combined extractor + screw.