New AT_ flags for restricting pathname lookup

eBPF is a nice thing to have if machine-generated (it's a rather nice and orthogonal assembler, and the ability to add helpers is just a killer feature that I wish real assemblers had!), but it's about as pleasant to debug programs written in it as any other assembler: i.e. fairly easy if you're familiar with the code generator, a nightmare otherwise, doubly so if this is the less regular land of handwritten code, disassembled and devoid of comments.