Implement decoding of preadv2 and pwritev2 syscalls

Implement decoding of preadv2 and pwritev2 syscalls

* io.c: Include "xlat/rwf_flags.h".
(do_preadv, do_pwritev, SYS_FUNC(preadv2), SYS_FUNC(pwritev2)):
New functions.
(SYS_FUNC(preadv)): Use do_preadv.
(SYS_FUNC(pwritev)): Use do_pwritev.
* linux/32/syscallent.h (preadv2, pwritev2): New entries.
* linux/64/syscallent.h: Likewise.
* linux/arm/syscallent.h: Likewise.
* linux/hppa/syscallent.h: Likewise.
* linux/i386/syscallent.h: Likewise.
* linux/ia64/syscallent.h: Likewise.
* linux/m68k/syscallent.h: Likewise.
* linux/mips/syscallent-n32.h: Likewise.
* linux/mips/syscallent-n64.h: Likewise.
* linux/mips/syscallent-o32.h: Likewise.
* linux/powerpc/syscallent.h: Likewise.
* linux/powerpc64/syscallent.h: Likewise.
* linux/s390/syscallent.h: Likewise.
* linux/s390x/syscallent.h: Likewise.
* linux/sparc/syscallent.h: Likewise.
* linux/sparc64/syscallent.h: Likewise.
* linux/x32/syscallent.h: Likewise.
* linux/x86_64/syscallent.h: Likewise.
* syscall.c (dumpio): Handle SEN_preadv2 and SEN_pwritev2.
* xlat/rwf_flags.in: New file.
* NEWS: Mention parsers of new syscalls.