tidy-up: drop stray comparisons with literal zero

Drop from:
- strcmp, strcmpi, strncmp, memcmp, lstat, getrlimit, setrlimit, fseek,
  fstat
- autotools detection snippets.
- smooth-gtk-thread: simplify `!var != 0` expression.

Closes #21947

lib/http_proxy.c

@@ -512,7 +512,7 @@ CURLcode Curl_http_proxy_inspect_tunnel_response(
       capsule_protocol = Curl_dynhds_cget(&resp->headers,
                                            "capsule-protocol");
       if(capsule_protocol) {
-        if(strncmp(capsule_protocol->value, "?1", 2) == 0 &&
+        if(!strncmp(capsule_protocol->value, "?1", 2) &&
            !capsule_protocol->value[2]) {
           infof(data, "CONNECT-UDP tunnel established, response %d",
                 resp->status);