Tap plugin

This PR adds a plugin to create tap devices.
The plugin adds a tap device to the container.

The plugin has a workaround for a golang netlink library
which does not allow for tap devices with no owner/group
to be created. When no tap owner/group is requested, the
plugin will fall back to using the ip tool for creating
the tap device. A fix to the golang netlink lib is pending.

Signed-off-by: mmirecki <mmirecki@redhat.com>

vendor/github.com/opencontainers/selinux/go-selinux/xattrs_linux.go

@@ -0,0 +1,38 @@
+package selinux
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// lgetxattr returns a []byte slice containing the value of
+// an extended attribute attr set for path.
+func lgetxattr(path, attr string) ([]byte, error) {
+	// Start with a 128 length byte array
+	dest := make([]byte, 128)
+	sz, errno := doLgetxattr(path, attr, dest)
+	for errno == unix.ERANGE {
+		// Buffer too small, use zero-sized buffer to get the actual size
+		sz, errno = doLgetxattr(path, attr, []byte{})
+		if errno != nil {
+			return nil, errno
+		}
+
+		dest = make([]byte, sz)
+		sz, errno = doLgetxattr(path, attr, dest)
+	}
+	if errno != nil {
+		return nil, errno
+	}
+
+	return dest[:sz], nil
+}
+
+// doLgetxattr is a wrapper that retries on EINTR
+func doLgetxattr(path, attr string, dest []byte) (int, error) {
+	for {
+		sz, err := unix.Lgetxattr(path, attr, dest)
+		if err != unix.EINTR {
+			return sz, err
+		}
+	}
+}