unix: add openat2 for linux

openat2 is a new syscall added to Linux 5.6. It provides a superset of
openat(2) functionality, extending it with flags telling the kernel how
to resolve the paths.

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

NOTE that this is a second attempt to add the call; the previous one
(https://golang.org/cl/227280) was reverted
(https://golang.org/cl/227846) due to the test case failure on ARM
(https://golang.org/issue/38357).

This CL has the test case reworked to be less assumptive to the testing
environment. In particular, it first tries if the most simplistic
openat2() call succeeds, and skips the test otherwise. It is done that
way because CI can be under under different kernels and in various
envrionments -- in particular, Docker+seccomp can result in EPERM from a
system call (which is not expected otherwise).

For previous discussions about the test case, see
https://golang.org/cl/227865.

Change-Id: I4276cf13dc29ecbdbdc9c58da0f76270f585a67f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/253057
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -136,6 +136,12 @@ 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) {