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/htmltitle.cpp

@@ -94,7 +94,7 @@ static bool init(CURL *&conn, const char *url)
 
   if(conn == NULL) {
     fprintf(stderr, "Failed to create CURL connection\n");
-    exit(EXIT_FAILURE);
+    return false;
   }
 
   code = curl_easy_setopt(conn, CURLOPT_ERRORBUFFER, errorBuffer);
@@ -270,7 +270,7 @@ int main(int argc, char *argv[])
 
   if(argc != 2) {
     fprintf(stderr, "Usage: %s <url>\n", argv[0]);
-    exit(EXIT_FAILURE);
+    return EXIT_FAILURE;
   }
 
   curl_global_init(CURL_GLOBAL_DEFAULT);
@@ -279,7 +279,7 @@ int main(int argc, char *argv[])
 
   if(!init(conn, argv[1])) {
     fprintf(stderr, "Connection initialization failed\n");
-    exit(EXIT_FAILURE);
+    return EXIT_FAILURE;
   }
 
   // Retrieve content for the URL
@@ -289,7 +289,7 @@ int main(int argc, char *argv[])
 
   if(code != CURLE_OK) {
     fprintf(stderr, "Failed to get '%s' [%s]\n", argv[1], errorBuffer);
-    exit(EXIT_FAILURE);
+    return EXIT_FAILURE;
   }
 
   // Parse the (assumed) HTML code