Kernel: *at syscalls should fully ignore dirfd if an absolute path is given

[timschumi]

At the moment, most *at syscalls will still try to resolve the directory custody before checking the path (with the latter happening in VirtualFileSystem::resolve_path_without_veil), so passing an invalid or non-directory file descriptor (such as 0, 1 or 2) will cause errors that shouldn't happen according to POSIX.

#23099 papered over this for the utimensat functionality in particular, but ideally there should be a structurally sound fix that we can easily apply to all similar syscalls. One potential solution is to not pass the resolved Custody down to resolve_path_without_veil, but rather a fallible lambda function that resolves the Custody on demand if required.