Sockets: declare msghdr->msg_iovlen as msg_iovlen_t

* Lwip declares msghdr->msg_iovlen as int, but when
  using external socket headers, some systems declare
  msg_iovlen as size_t or others.
* This patch creates a new type msg_iovlen_t and
  expects users to typedef it to the type they need
  for their system.

src/include/lwip/sockets.h

@@ -131,11 +131,13 @@ struct iovec {
 };
 #endif
 
+typedef int msg_iovlen_t;
+
 struct msghdr {
   void         *msg_name;
   socklen_t     msg_namelen;
   struct iovec *msg_iov;
-  int           msg_iovlen;
+  msg_iovlen_t  msg_iovlen;
   void         *msg_control;
   socklen_t     msg_controllen;
   int           msg_flags;