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 3829759bd0 #12489
Closes #13501

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