New AT_ flags for restricting pathname lookup

Something like resolveat(2)? The problem is that this would necessarily be conceptually identical to openat(O_PATH). Maybe O_PATH should've been a different syscall but we are mostly stuck with it now, and I think it would be strange to have two methods of opening an O_PATH descriptor. Though, there are some aspects of O_PATH that I think need to be fixed (and would require more convoluted O_ flags -- maybe a new syscall is warranted to fix some of the semantics of O_PATH. I'm not sure.)

And remember that the widespread utility of any resolveat(2) syscall would likely require having AT_EMPTY_PATH support for every *at(2) syscall (which is unfortunately far from the case currently).