Add queue.h tree.h _null.h

Import queue.h tree.h _null.h header files from OpenBSD upstream
since x509_issuer_cache.c requires them.

include/compat/sys/_null.h

@@ -0,0 +1,18 @@
+/*	$OpenBSD: _null.h,v 1.2 2016/09/09 22:07:58 millert Exp $	*/
+
+/*
+ * Written by Todd C. Miller, September 9, 2016
+ * Public domain.
+ */
+
+#ifndef NULL
+#if !defined(__cplusplus)
+#define	NULL	((void *)0)
+#elif __cplusplus >= 201103L
+#define	NULL	nullptr
+#elif defined(__GNUG__)
+#define	NULL	__null
+#else
+#define	NULL	0L
+#endif
+#endif