unix: add openat2 for linux

openat2 is a new syscall added to Linux 5.6. It provides a superset
of openat(2) functionality, providing a way to extend it in the future,
and (for now) adding flags telling the kernel how to resolve the path.

For more info on openat2, see https://lwn.net/Articles/803237/

A primitive test case is added to check that Openat2 works as
it should. Tested to skip on kernel 5.5 and pass on 5.6.

Change-Id: Ib8bbd71791762f043200543cecdea16d2fd3c81d
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/sys/+/227280
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -133,6 +133,12 @@ func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error)
 	return openat(dirfd, path, flags|O_LARGEFILE, mode)
 }
 
+//sys	openat2(dirfd int, path string, open_how *OpenHow, size int) (fd int, err error)
+
+func Openat2(dirfd int, path string, how *OpenHow) (fd int, err error) {
+	return openat2(dirfd, path, how, SizeofOpenHow)
+}
+
 //sys	ppoll(fds *PollFd, nfds int, timeout *Timespec, sigmask *Sigset_t) (n int, err error)
 
 func Ppoll(fds []PollFd, timeout *Timespec, sigmask *Sigset_t) (n int, err error) {