core: use getdents64 syscall on linux instead of getdents.

* src/posix-io.c (get_max_fds): use getdents64 instead of getdents.

getdents64 was introduced in linux 2.4, so it should be widely
available. some Linux architectures which post-date 2.4 (e.g. arm64)
appear to not have getdents at all, so it's probably better to use the
more modern interface.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>