tidy-up: prefer return over exit(), fix fallouts

To avoid breaking the control flow and align to majority of code
already using `return`.

`exit()` has the side-effect of suppressing leak detection in cases.
Fix fallouts detected after switching to `return`.

- configure:
  - fix `getaddrinfo` run test to call `freeaddrinfo()` to pacify ASAN,
    and call `WSACleanup()` to deinit winsock2.
  - fix `getifaddrs` run test to call `freeifaddrs()` to pacify ASAN.
- tests/server:
  - setup `atexit(win32_cleanup)` via `win32_init()`.
  - return 2 instead of 1 on winsock2 init failures.
  - sws: goto cleanup instead of `exit()` in `http_connect()`.
    Follow-up to 02dfe71937 #7235
- tests/client/http:
  - cleanup memory to pacify ASAN in `h2-upgrade-extreme`,
    `tls-session-reuse`.
- examples:
  - block_ip: fix memory leak reported by CI.
  - http2-upload: avoid handle leaks.

Untouched `exit()` calls, made from callbacks:
- docs/examples: ephiperfifo.c, ghiper.c, hiperfifo.c
- tests/libtest: lib582.c, lib655.c, lib670.c
- tests/server: tftpd.c

Closes #16507

docs/examples/block_ip.c

@@ -298,14 +298,19 @@ int main(void)
 
   filter = (struct connection_filter *)calloc(1, sizeof(*filter));
   if(!filter)
-    exit(1);
+    return 1;
 
-  if(curl_global_init(CURL_GLOBAL_DEFAULT))
-    exit(1);
+  if(curl_global_init(CURL_GLOBAL_DEFAULT)) {
+    free(filter);
+    return 1;
+  }
 
   curl = curl_easy_init();
-  if(!curl)
-    exit(1);
+  if(!curl) {
+    curl_global_cleanup();
+    free(filter);
+    return 1;
+  }
 
   /* Set the target URL */
   curl_easy_setopt(curl, CURLOPT_URL, "http://localhost");