examples: fix/silence -Wsign-conversion

- extend `FD_SET()` hack to all platforms (was only Cygwin).
  Warnings may also happen in other envs, e.g. OmniOS.
  Ref: https://github.com/libssh2/libssh2/actions/runs/8854199687/job/24316762831#step:3:2021

- tidy-up `CURLcode` vs `int` use.

- cast an unsigned to `long` before passing to `curl_easy_setopt()`.

Cherry-picked from #13489
Follow-up to 3829759 #12489
Closes #13501

docs/examples/address-scope.c

@@ -44,7 +44,7 @@ int main(void)
     long my_scope_id;
     curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
 
-    my_scope_id = if_nametoindex("eth0");
+    my_scope_id = (long)if_nametoindex("eth0");
     curl_easy_setopt(curl, CURLOPT_ADDRESS_SCOPE, my_scope_id);
 
     /* Perform the request, res gets the return code */

docs/examples/ftp-wildcard.c

@@ -50,9 +50,9 @@ int main(int argc, char **argv)
   struct callback_data data = { 0 };
 
   /* global initialization */
-  int rc = curl_global_init(CURL_GLOBAL_ALL);
+  CURLcode rc = curl_global_init(CURL_GLOBAL_ALL);
   if(rc)
-    return rc;
+    return (int)rc;
 
   /* initialization of easy handle */
   handle = curl_easy_init();
@@ -90,7 +90,7 @@ int main(int argc, char **argv)
 
   curl_easy_cleanup(handle);
   curl_global_cleanup();
-  return rc;
+  return (int)rc;
 }
 
 static long file_is_coming(struct curl_fileinfo *finfo,

docs/examples/sendrecv.c

@@ -48,7 +48,7 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
  * warning: conversion to 'long unsigned int' from 'curl_socket_t' {aka 'int'}
  * may change the sign of the result [-Wsign-conversion]
  */
-#if defined(__GNUC__) && defined(__CYGWIN__)
+#if defined(__GNUC__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
@@ -60,7 +60,7 @@ static int wait_on_socket(curl_socket_t sockfd, int for_recv, long timeout_ms)
   else {
     FD_SET(sockfd, &outfd);
   }
-#if defined(__GNUC__) && defined(__CYGWIN__)
+#if defined(__GNUC__)
 #pragma GCC diagnostic pop
 #endif
 

docs/examples/websocket.c

@@ -68,16 +68,13 @@ static int recv_pong(CURL *curl, const char *expected_payload)
   return (int)result;
 }
 
-static int recv_any(CURL *curl)
+static CURLcode recv_any(CURL *curl)
 {
   size_t rlen;
   const struct curl_ws_frame *meta;
   char buffer[256];
-  CURLcode result = curl_ws_recv(curl, buffer, sizeof(buffer), &rlen, &meta);
-  if(result)
-    return result;
 
-  return 0;
+  return curl_ws_recv(curl, buffer, sizeof(buffer), &rlen, &meta);
 }
 
 /* close the connection */