Revert "cygwin: only expose /dev/con{in,out,sole} when started from a Windows console"

This caused serious regressions when running from a cmd window:

https://cygwin.com/ml/cygwin/2017-09/msg00114.html

This reverts commit b706c6b479.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/fhandler.h

@@ -1439,7 +1439,7 @@ private:
   ssize_t __stdcall write (const void *ptr, size_t len);
   void doecho (const void *str, DWORD len) { (void) write (str, len); }
   int close ();
-  static bool exists () {return shared_console_info && !!GetConsoleCP ();}
+  static bool exists () {return !!GetConsoleCP ();}
 
   int tcflush (int);
   int tcsetattr (int a, const struct termios *t);