eio_linux: fallback to fork if clone3 is unavailable

[talex5]

clone3 is blocked by Docker's default security policy.

Also, use (uintptr_t) when storing a pointer in a uint64_t field, otherwise it doesn't work on 32-bit systems.

Fixes #514.

[avsm]

This does leave us in a strange half-way house, since we don't derive any benefit from using clone3 if the interface always must be able to fallback to fork(). Any reason not to always just use fork?

[talex5]

Any reason not to always just use fork?

I was going to do that, but the fallback path requires some awkward code to handle the case where you spawn a process and then can't get an FD for it (e.g. due to FD limits), so I decided to use the new system when available. Also, I'm still planning to add support for namespaces at some point (it's OK if that doesn't work in a container).