Make <libutil.h> work when included by itself.

There are several reasons why it didn't work:

- It was missing <sys/cdefs.h> for __BEGIN_DECLS.
- It uses various primitive types that were not declared.

lib/libutil/libutil.h

@@ -39,6 +39,34 @@
 #ifndef _LIBUTIL_H_
 #define	_LIBUTIL_H_
 
+#include <sys/cdefs.h>
+#include <sys/_types.h>
+
+#ifndef _GID_T_DECLARED
+typedef	__gid_t		gid_t;
+#define	_GID_T_DECLARED
+#endif
+
+#ifndef _INT64_T_DECLARED
+typedef	__int64_t	int64_t;
+#define	_INT64_T_DECLARED
+#endif
+
+#ifndef _PID_T_DECLARED
+typedef	__pid_t		pid_t;
+#define	_PID_T_DECLARED
+#endif
+
+#ifndef _SIZE_T_DECLARED
+typedef	__size_t	size_t;
+#define	_SIZE_T_DECLARED
+#endif
+
+#ifndef _UID_T_DECLARED
+typedef	__uid_t		uid_t;
+#define	_UID_T_DECLARED
+#endif
+
 #define PROPERTY_MAX_NAME	64
 #define PROPERTY_MAX_VALUE	512