Clean up <stdlib.h> slightly.

Interestingly, this mostly involves cleaning up our implementation of
various <string.h> functions.

Change-Id: Ifaef49b5cb997134f7bc0cc31bdac844bdb9e089

libc/include/string.h

@@ -36,6 +36,10 @@
 
 __BEGIN_DECLS
 
+#if defined(__USE_BSD)
+#include <strings.h>
+#endif
+
 extern void*  memccpy(void* __restrict, const void* __restrict, int, size_t);
 extern void*  memchr(const void *, int, size_t) __purefunc;
 extern void*  memrchr(const void *, int, size_t) __purefunc;