Re-vendor, primarily for https://github.com/containers/storage/pull/11

containers/storage got new dependencies, so we will need to re-vendor
eventually anyway, and having this separate from other major work is
cleaner.

But the primary goal of this commit is to see whether it makes skopeo
buildable on OS X.

vendor/golang.org/x/sys/unix/sockcmsg_linux.go

@@ -0,0 +1,36 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Socket control messages
+
+package unix
+
+import "unsafe"
+
+// UnixCredentials encodes credentials into a socket control message
+// for sending to another process. This can be used for
+// authentication.
+func UnixCredentials(ucred *Ucred) []byte {
+	b := make([]byte, CmsgSpace(SizeofUcred))
+	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
+	h.Level = SOL_SOCKET
+	h.Type = SCM_CREDENTIALS
+	h.SetLen(CmsgLen(SizeofUcred))
+	*((*Ucred)(cmsgData(h))) = *ucred
+	return b
+}
+
+// ParseUnixCredentials decodes a socket control message that contains
+// credentials in a Ucred structure. To receive such a message, the
+// SO_PASSCRED option must be enabled on the socket.
+func ParseUnixCredentials(m *SocketControlMessage) (*Ucred, error) {
+	if m.Header.Level != SOL_SOCKET {
+		return nil, EINVAL
+	}
+	if m.Header.Type != SCM_CREDENTIALS {
+		return nil, EINVAL
+	}
+	ucred := *(*Ucred)(unsafe.Pointer(&m.Data[0]))
+	return &ucred, nil
+}