wincap.h: Rename assitant to assistant throughout. wincap.cc: Ditto.

* devices.in (exists_console): Use fhandler_console::exists () rather than raw
test.
* devices.cc: Regenerate.
* fhandler.h (fhandler_console::exists): Define new function.
* fhandler_console.cc (fhandler_console::need_invisible): Use
fhandler_console::exists () rather than raw test.
* spawn.cc: Rename assitant to assistant throughout.
(child_info_spawn::worker): Simplify test for when to start a non-Cygwin
process in its own process group.  Just do it whenever we start a non-Cygwin
process.

winsup/cygwin/fhandler.h

@@ -1377,6 +1377,7 @@ 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 !!GetConsoleCP ();}
 
   int tcflush (int);
   int tcsetattr (int a, const struct termios *t);