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/input/input_event.h

@@ -4,7 +4,6 @@
 #include <functional>
 
 #include <torrent/event.h>
-#include <torrent/system/poll.h>
 
 namespace input {
 
@@ -16,8 +15,8 @@ public:
 
   const char*         type_name() const override { return "input"; }
 
-  void                insert(torrent::system::Poll* p);
-  void                remove(torrent::system::Poll* p);
+  void                insert();
+  void                remove();
 
   void                event_read() override;
   void                event_write() override;