Simplify sendmmsg code

[Florian Weimer]

On 05/10/2017 02:14 PM, Adhemerval Zanella wrote:
> Indeed the emulation brings a lot of issue, but it allows us to use the system
> specific syscalls on generic code without relying on additional checks for
> functionality existence.  However I think we can to Linux optimizations in
> generic code less intrusive way if necessary with a different approach, maybe
> by implementing the functionally in a more constrained way and reimplement
> in the target system.

Yes, optimizations are fine.  But I think it is wrong to expose the 
fallback implementation to applications as an implementation of the 
system call, when in fact the semantics are different.  Every time we do 
this, it comes back to haunt as eventually (see posix_fallocate or 
euidaccess for examples).

Thanks,
Florian