rearrange to allow internal/private use of ares_writev to any system

that lacks the writev function.

ares_writev.h

@@ -0,0 +1,37 @@
+#ifndef HEADER_CARES_WRITEV_H
+#define HEADER_CARES_WRITEV_H
+
+/* $Id$ */
+
+/* Copyright 1998 by the Massachusetts Institute of Technology.
+ *
+ * Permission to use, copy, modify, and distribute this
+ * software and its documentation for any purpose and without
+ * fee is hereby granted, provided that the above copyright
+ * notice appear in all copies and that both that copyright
+ * notice and this permission notice appear in supporting
+ * documentation, and that the name of M.I.T. not be used in
+ * advertising or publicity pertaining to distribution of the
+ * software without specific, written prior permission.
+ * M.I.T. makes no representations about the suitability of
+ * this software for any purpose.  It is provided "as is"
+ * without express or implied warranty.
+ */
+
+#include "setup.h"
+#include "ares.h"
+
+#ifndef HAVE_WRITEV
+
+/* Structure for scatter/gather I/O. */
+struct iovec
+{
+  void *iov_base;  /* Pointer to data. */
+  size_t iov_len;  /* Length of data.  */
+};
+
+extern ssize_t ares_writev(ares_socket_t s, const struct iovec *iov, int iovcnt);
+
+#endif
+
+#endif /* HEADER_CARES_WRITEV_H */