Separate process wait and close

On Linux, waitpid() both waits for the process to terminate and reaps it
(closes its handle). On Windows, these actions are separated into
WaitForSingleObject() and CloseHandle().

Expose these actions separately, so that it is possible to send a signal
to a process while waiting for its termination without race condition.

This allows to wait for server termination normally, but kill the
process without race condition if it is not terminated after some delay.

app/src/server.c

@@ -392,7 +392,7 @@ server_init(struct server *server) {
 static int
 run_wait_server(void *data) {
     struct server *server = data;
-    process_wait(server->process, NULL); // ignore exit code
+    process_wait_noclose(server->process, NULL); // ignore exit code
 
     mutex_lock(server->mutex);
     server->process_terminated = true;
@@ -550,14 +550,14 @@ server_stop(struct server *server) {
     // On some devices, closing the sockets is not sufficient to wake up the
     // blocking calls while the device is asleep.
     if (r == SDL_MUTEX_TIMEDOUT) {
-        // FIXME There is a race condition here: there is a small chance that
-        // the process is already terminated, and the PID assigned to a new
-        // process.
+        // The process is terminated, but not reaped (closed) yet, so its PID
+        // is still valid.
         LOGW("Killing the server...");
         process_terminate(server->process);
     }
 
     SDL_WaitThread(server->wait_server_thread, NULL);
+    process_close(server->process);
 }
 
 void