Include string.h unconditionally and only use strings.h for strn?casecmp()

In the pre-POSIX days BSD had strings.h, not string.h.
Now strings.h is only used for non-ANSI string functions.

lib/util/mkdir_parents.c

@@ -31,12 +31,7 @@
 #else
 # include "compat/stdbool.h"
 #endif /* HAVE_STDBOOL_H */
-#ifdef HAVE_STRING_H
-# include <string.h>
-#endif /* HAVE_STRING_H */
-#ifdef HAVE_STRINGS_H
-# include <strings.h>
-#endif /* HAVE_STRINGS_H */
+#include <string.h>
 #include <unistd.h>
 #include <errno.h>
 #include <fcntl.h>