[PATCH] posix: Add p{read,write}v2 RWF_NOWAIT flag (BZ#21738)

[Florian Weimer]

* Adhemerval Zanella:

> +@item RWF_SUPPORTED
> +Mask with all supported flags.

I assume the kernel intends to add further flags in the future.  I'm
not sure it's wise to provide this to applications because it's not
very likely that it's going to match the flag mask used by the kernel.