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_tty.cc

@@ -525,7 +525,7 @@ fhandler_pty_master::accept_input ()
     }
   else
     {
-      char *p = rabuf;
+      char *p = rabuf ();
       DWORD rc;
       DWORD written = 0;