Linux: Add `preadv2` and `pwritev2` and associated constants
Linux: Add `preadv2` and `pwritev2` and associated constants

These functions are the same as `preadv` and `pwritev` but have a flags
parameter.  `preadv2()` and `pwritev2()` first appeared in Linux 4.6.
Library support was added in glibc 2.26.

rust-lang/libc,rust-lang/libc,rust-lang/libc