Linux: implement decoding of preadv and pwritev syscalls

* io.c [LINUX && HAVE_SYS_UIO_H] (sys_preadv, sys_pwritev): New functions.
* linux/syscall.h (sys_preadv, sys_pwritev): Declare them.
* linux/*/syscallent.h: Use them.

Signed-off-by: Damir Shayhutdinov <damir@altlinux.ru>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
18 files changed