Use memfd_create on Linux once it becomes available in libc

[gnzlbg]

This call to mkstemp:

https://github.com/gnzlbg/slice_deque/blob/master/src/mirrored/linux.rs#L40

should be replaced by a memfd_create call. When Linus Torvalds recommended using mkstemp here:

I appreciate the carefulness of this, but why not just do

    `static char filename[] = "/tmp/DoubleMapXXXXXX";`
   ` int fd = mkstemp(filename);`

which is rather simpler and does much less work

memfd_create did not exist yet. The first step is:

• Finish my couple of weeks old PR to libc that adds memfd_create: rust-lang/libc#902

However, I'll probably end up stripping memfd_create out-of-it because it is not exposed by glibc, so it shouldn't be exposed by libc either.

• We will need to either find a crate that exposes it (maybe use the syscall.rs crate ?), or create a small crate that just exposes it and add it as a linux dependency.

pinging @bill-myers because he is also interested in seeing this fixed