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/http2-download.c

@@ -142,7 +142,7 @@ int my_trace(CURL *handle, curl_infotype type,
   return 0;
 }
 
-static void setup(struct transfer *t, int num)
+static int setup(struct transfer *t, int num)
 {
   char filename[128];
   CURL *hnd;
@@ -155,7 +155,7 @@ static void setup(struct transfer *t, int num)
   if(!t->out) {
     fprintf(stderr, "error: could not open file %s for writing: %s\n",
             filename, strerror(errno));
-    exit(1);
+    return 1;
   }
 
   /* write to this file */
@@ -179,6 +179,7 @@ static void setup(struct transfer *t, int num)
   /* wait for pipe connection to confirm */
   curl_easy_setopt(hnd, CURLOPT_PIPEWAIT, 1L);
 #endif
+  return 0;
 }
 
 /*
@@ -204,7 +205,8 @@ int main(int argc, char **argv)
   multi_handle = curl_multi_init();
 
   for(i = 0; i < num_transfers; i++) {
-    setup(&trans[i], i);
+    if(setup(&trans[i], i))
+      return 1;
 
     /* add the individual transfer */
     curl_multi_add_handle(multi_handle, trans[i].easy);