input: survive controlling-terminal/pty hangup on stdin

Since the 0.16.13 callback/poll rework, an EPOLLERR on stdin (controlling
terminal or pty hangup) reaches Poll::process(), which aborts the whole
client with an internal_error because InputEvent never registered for error
events:

    Poll::process() received error event for event not in error: input-fd:0

Register stdin for error events (insert_error) and handle event_error() by
dropping stdin from the poll set with this_thread::poll()->remove_and_close().
rtorrent then keeps running without keyboard input instead of dying.

remove() guards on the fd state (is_open()), mirroring SCgiTask: event_error()
clears the fd after remove_and_close(), and the shutdown path (Control::cleanup)
still calls remove(); without the guard the second remove_and_close() throws
'event not found' via event_mask(). insert()/remove() take the thread poll
implicitly (this_thread::poll()) instead of a Poll* argument.

src/control.cc

@@ -76,7 +76,7 @@ Control::initialize() {
   m_ui->init(this);
 
   if(!display::Canvas::daemon())
-    m_inputStdin->insert(torrent::this_thread::poll());
+    m_inputStdin->insert();
 }
 
 void
@@ -86,7 +86,7 @@ Control::cleanup() {
   torrent::this_thread::scheduler()->erase(&m_task_shutdown);
 
   if(!display::Canvas::daemon())
-    m_inputStdin->remove(torrent::this_thread::poll());
+    m_inputStdin->remove();
 
   if (scgi_thread::thread()->is_active())
     scgi_thread::thread()->stop_thread_wait();