Cygwin: console: Share readahead buffer within the same process.

- The cause of the problem reported in
  https://www.cygwin.com/ml/cygwin/2020-01/msg00220.html is that the
  chars input before dup() cannot be read from the new file descriptor.
  This is because the readahead buffer (rabuf) in the console is newly
  created by dup(), and does not inherit from the parent. This patch
  fixes the issue.

winsup/cygwin/fhandler.h

@@ -202,16 +202,21 @@ class fhandler_base
 
   ino_t ino;	/* file ID or hashed filename, depends on FS. */
   LONG _refcnt;
+ public:
+  struct rabuf_t
+  {
+    char *rabuf;		/* used for crlf conversion in text files */
+    size_t ralen;
+    size_t raixget;
+    size_t raixput;
+    size_t rabuflen;
+  };
 
  protected:
   /* File open flags from open () and fcntl () calls */
   int openflags;
 
-  char *rabuf;		/* used for crlf conversion in text files */
-  size_t ralen;
-  size_t raixget;
-  size_t raixput;
-  size_t rabuflen;
+  struct rabuf_t ra;
 
   /* Used for advisory file locking.  See flock.cc.  */
   int64_t unique_id;
@@ -315,7 +320,13 @@ class fhandler_base
     ReleaseSemaphore (read_state, n, NULL);
   }
 
-  bool get_readahead_valid () { return raixget < ralen; }
+  virtual char *&rabuf () { return ra.rabuf; };
+  virtual size_t &ralen () { return ra.ralen; };
+  virtual size_t &raixget () { return ra.raixget; };
+  virtual size_t &raixput () { return ra.raixput; };
+  virtual size_t &rabuflen () { return ra.rabuflen; };
+
+  bool get_readahead_valid () { return raixget () < ralen (); }
   int puts_readahead (const char *s, size_t len = (size_t) -1);
   int put_readahead (char value);
 
@@ -464,8 +475,8 @@ public:
   virtual bg_check_types bg_check (int, bool = false) {return bg_ok;}
   virtual void clear_readahead ()
   {
-    raixput = raixget = ralen = rabuflen = 0;
-    rabuf = NULL;
+    raixput () = raixget () = ralen () = rabuflen () = 0;
+    rabuf () = NULL;
   }
   void operator delete (void *p) {cfree (p);}
   virtual void set_eof () {}
@@ -2051,6 +2062,12 @@ private:
   DWORD __acquire_output_mutex (const char *fn, int ln, DWORD ms);
   void __release_output_mutex (const char *fn, int ln);
 
+  char *&rabuf ();
+  size_t &ralen ();
+  size_t &raixget ();
+  size_t &raixput ();
+  size_t &rabuflen ();
+
   friend tty_min * tty_list::get_cttyp ();
 };