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.

test/unit/api/test_sockets.c

@@ -250,7 +250,7 @@ static void test_sockets_init_loopback_addr(int domain, struct sockaddr_storage
 
 static void test_sockets_msgapi_update_iovs(struct msghdr *msg, size_t bytes)
 {
-  int i;
+  msg_iovlen_t i;
 
   /* note: this modifies the underyling iov_base and iov_len for a partial
      read for an individual vector. This updates the msg->msg_iov pointer