url: fix logic in connection reuse to deny reuse on "unclean" connections

- add parameter to `conn_is_alive()` cfilter method that returns
  if there is input data waiting on the connection
- refrain from re-using connnection from the cache that have
  input pending
- adapt http/2 and http/3 alive checks to digest pending input
  to check the connection state
- remove check_cxn method from openssl as that was just doing
  what the socket filter now does.
- add tests for connection reuse with special server configs

Closes #10690

lib/cf-socket.c

@@ -325,20 +325,6 @@ int Curl_socket_close(struct Curl_easy *data, struct connectdata *conn,
   return socket_close(data, conn, FALSE, sock);
 }
 
-bool Curl_socket_is_dead(curl_socket_t sock)
-{
-  int sval;
-  bool ret_val = TRUE;
-
-  sval = SOCKET_READABLE(sock, 0);
-  if(sval == 0)
-    /* timeout */
-    ret_val = FALSE;
-
-  return ret_val;
-}
-
-
 #ifdef USE_WINSOCK
 /* When you run a program that uses the Windows Sockets API, you may
    experience slow performance when you copy data to a TCP server.
@@ -1449,12 +1435,14 @@ static CURLcode cf_socket_cntrl(struct Curl_cfilter *cf,
 }
 
 static bool cf_socket_conn_is_alive(struct Curl_cfilter *cf,
-                                    struct Curl_easy *data)
+                                    struct Curl_easy *data,
+                                    bool *input_pending)
 {
   struct cf_socket_ctx *ctx = cf->ctx;
   struct pollfd pfd[1];
   int r;
 
+  *input_pending = FALSE;
   (void)data;
   if(!ctx || ctx->sock == CURL_SOCKET_BAD)
     return FALSE;
@@ -1479,6 +1467,7 @@ static bool cf_socket_conn_is_alive(struct Curl_cfilter *cf,
   }
 
   DEBUGF(LOG_CF(data, cf, "is_alive: valid events, looks alive"));
+  *input_pending = TRUE;
   return TRUE;
 }