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_unix.c

@@ -522,8 +522,6 @@ void mch_exit(int r)
 {
   exiting = TRUE;
 
-  event_teardown();
-
   {
     settmode(TMODE_COOK);
     mch_restore_title(3);       /* restore xterm title and icon name */
@@ -559,7 +557,7 @@ void mch_exit(int r)
   mac_conv_cleanup();
 #endif
 
-
+  event_teardown();
 
 #ifdef EXITFREE
   free_all_mem();