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_console.cc

@@ -56,6 +56,10 @@ bool NO_COPY fhandler_console::invisible_console;
 static DWORD orig_conin_mode = (DWORD) -1;
 static DWORD orig_conout_mode = (DWORD) -1;
 
+/* con_ra is shared in the same process.
+   Only one console can exist in a process, therefore, static is suitable. */
+static struct fhandler_base::rabuf_t con_ra;
+
 static void
 beep ()
 {
@@ -214,6 +218,36 @@ fhandler_console::setup ()
     }
 }
 
+char *&
+fhandler_console::rabuf ()
+{
+  return con_ra.rabuf;
+}
+
+size_t &
+fhandler_console::ralen ()
+{
+  return con_ra.ralen;
+}
+
+size_t &
+fhandler_console::raixget ()
+{
+  return con_ra.raixget;
+}
+
+size_t &
+fhandler_console::raixput ()
+{
+  return con_ra.raixput;
+}
+
+size_t &
+fhandler_console::rabuflen ()
+{
+  return con_ra.rabuflen;
+}
+
 /* Return the tty structure associated with a given tty number.  If the
    tty number is < 0, just return a dummy record. */
 tty_min *
@@ -454,7 +488,7 @@ fhandler_console::read (void *pv, size_t& buflen)
   copied_chars +=
     get_readahead_into_buffer (buf + copied_chars, buflen);
 
-  if (!ralen)
+  if (!con_ra.ralen)
     input_ready = false;
   release_input_mutex ();
 
@@ -1103,6 +1137,9 @@ fhandler_console::close ()
   CloseHandle (get_handle ());
   CloseHandle (get_output_handle ());
 
+  if (con_ra.rabuf)
+    free (con_ra.rabuf);
+
   /* If already attached to pseudo console, don't call free_console () */
   cygheap_fdenum cfd (false);
   while (cfd.next () >= 0)