event: Ensure the event loop has been cleaned up in event_teardown

- Add input_teardown/signal_teardown to take care of closing signal/stdin
  handles.
- Call those functions in event_teardown, and ensure there are no active handles
  by entering an infinite loop when there are unclosed handles(think of this as
  an assertion that can't go unoticed on travis).
- Move event_teardown call to the end of mch_exit. That is required because
  event_poll may still be called in that function.

src/nvim/os/signal.c

@@ -55,6 +55,20 @@ void signal_init(void)
 #endif
 }
 
+void signal_teardown(void)
+{
+  signal_stop();
+  uv_close((uv_handle_t *)&sint, NULL);
+  uv_close((uv_handle_t *)&spipe, NULL);
+  uv_close((uv_handle_t *)&shup, NULL);
+  uv_close((uv_handle_t *)&squit, NULL);
+  uv_close((uv_handle_t *)&sterm, NULL);
+  uv_close((uv_handle_t *)&swinch, NULL);
+#ifdef SIGPWR
+  uv_close((uv_handle_t *)&spwr, NULL);
+#endif
+}
+
 void signal_stop(void)
 {
   uv_signal_stop(&sint);