define struct clone_args for linux-lts versions that don't have it

[copy]

I've chosen to define this struct unconditionally, and rename it:

• Renaming is necessary because, if we redefine struct clone_args, we'll get a compilation error if sched.h contains an older version of this struct
• I believe it's not possible to detect if a struct is defined using just preprocessor macros, so a configure script would be needed
• CLONE_PIDFD is not defined in the linux-lts headers, but it is defined in musl, so it can't be used to determine if struct clone_args is defined
• The Linux syscall interface is backwards-compatible anyway

But I'm open to suggestions for other fixes.

Fixes #737

[copy]

I realised that the original version caused a regression: Using the latest version of the struct unconditionally would break older kernels (at runtime). I've instead changed it to use the first version of the struct. The current code doesn't use any of the new fields (set_tid, set_tid_size and cgroup). If you'd like to check, sizeof caml_eio_clone_args now matches CLONE_ARGS_SIZE_VER0 from linux/sched.h.

[talex5]

Looks great - thanks!