Cygwin: pty: add pseudo console support.

- Support pseudo console in PTY. Pseudo console is a new feature
  in Windows 10 1809, which provides console APIs on virtual
  terminal. With this patch, native console applications can work
  in PTYs such as mintty, ssh, gnu screen or tmux.

winsup/cygwin/fhandler.h

@@ -2019,6 +2019,7 @@ private:
   static bool need_invisible ();
   static void free_console ();
   static const char *get_nonascii_key (INPUT_RECORD& input_rec, char *);
+  static DWORD get_console_process_id (DWORD pid, bool match);
 
   fhandler_console (void *) {}
 
@@ -2051,8 +2052,8 @@ class fhandler_pty_common: public fhandler_termios
  public:
   fhandler_pty_common ()
     : fhandler_termios (),
-      output_mutex (NULL),
-    input_mutex (NULL), input_available_event (NULL)
+    output_mutex (NULL), input_mutex (NULL),
+    input_available_event (NULL)
   {
     pc.file_attributes (FILE_ATTRIBUTE_NORMAL);
   }
@@ -2089,14 +2090,29 @@ class fhandler_pty_common: public fhandler_termios
     return fh;
   }
 
+  bool attach_pcon_in_fork (void)
+  {
+    return get_ttyp ()->attach_pcon_in_fork;
+  }
+  DWORD getHelperProcessId (void)
+  {
+    return get_ttyp ()->HelperProcessId;
+  }
+  HPCON getPseudoConsole (void)
+  {
+    return get_ttyp ()->hPseudoConsole;
+  }
+
  protected:
-  BOOL process_opost_output (HANDLE h, const void *ptr, ssize_t& len, bool is_echo);
+  BOOL process_opost_output (HANDLE h,
+			     const void *ptr, ssize_t& len, bool is_echo);
+  bool check_switch_to_pcon (void);
 };
 
 class fhandler_pty_slave: public fhandler_pty_common
 {
   HANDLE inuse;			// used to indicate that a tty is in use
-  HANDLE output_handle_cyg;
+  HANDLE output_handle_cyg, io_handle_cyg;
 
   /* Helper functions for fchmod and fchown. */
   bool fch_open_handles (bool chown);
@@ -2106,9 +2122,13 @@ class fhandler_pty_slave: public fhandler_pty_common
  public:
   /* Constructor */
   fhandler_pty_slave (int);
+  /* Destructor */
+  ~fhandler_pty_slave ();
 
   void set_output_handle_cyg (HANDLE h) { output_handle_cyg = h; }
   HANDLE& get_output_handle_cyg () { return output_handle_cyg; }
+  void set_handle_cyg (HANDLE h) { io_handle_cyg = h; }
+  HANDLE& get_handle_cyg () { return io_handle_cyg; }
 
   int open (int flags, mode_t mode = 0);
   void open_setup (int flags);
@@ -2149,6 +2169,15 @@ class fhandler_pty_slave: public fhandler_pty_common
     copyto (fh);
     return fh;
   }
+  void set_switch_to_pcon (void);
+  void reset_switch_to_pcon (void);
+  void push_to_pcon_screenbuffer (const char *ptr, size_t len);
+  bool has_master_opened (void);
+  void mask_switch_to_pcon (bool mask)
+  {
+    get_ttyp ()->mask_switch_to_pcon = mask;
+  }
+  void fixup_after_attach (bool native_maybe);
 };
 
 #define __ptsname(buf, unit) __small_sprintf ((buf), "/dev/pty%d", (unit))
@@ -2157,17 +2186,17 @@ class fhandler_pty_master: public fhandler_pty_common
   int pktmode;			// non-zero if pty in a packet mode.
   HANDLE master_ctl;		// Control socket for handle duplication
   cygthread *master_thread;	// Master control thread
-  HANDLE from_master, to_master;
+  HANDLE from_master, to_master, from_slave, to_slave;
   HANDLE echo_r, echo_w;
   DWORD dwProcessId;		// Owner of master handles
-  HANDLE io_handle_cyg, to_master_cyg;
+  HANDLE to_master_cyg, from_master_cyg;
   cygthread *master_fwd_thread;	// Master forwarding thread
 
 public:
   HANDLE get_echo_handle () const { return echo_r; }
-  HANDLE& get_handle_cyg () { return io_handle_cyg; }
   /* Constructor */
   fhandler_pty_master (int);
+  ~fhandler_pty_master ();
 
   DWORD pty_master_thread ();
   DWORD pty_master_fwd_thread ();
@@ -2212,6 +2241,8 @@ public:
     copyto (fh);
     return fh;
   }
+
+  bool setup_pseudoconsole (void);
 };
 
 class fhandler_dev_null: public fhandler_base