Cygwin: reduce size of fhandler_cygdrive

fhandler_cygdrive has a size of 696 bytes on x86_64, while the next
biggest fhandler type, fhandler_pty_master, is 584 bytes.  The members
responsible for the size are private to opendir/readdir/closedir usage.
fhandler_disk_file stores private readdir data in DIR->__d_internal
instead.  Use equivalent method with fhandler_cygdrive.  This drops
the size to 464 bytes.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -1350,14 +1350,6 @@ public:
 
 class fhandler_cygdrive: public fhandler_disk_file
 {
-  enum
-  {
-    DRVSZ = sizeof ("x:\\")
-  };
-  int ndrives;
-  const char *pdrive;
-  char pdrive_buf[1 + (2 * 26 * DRVSZ)];
-  void set_drives ();
  public:
   fhandler_cygdrive ();
   int open (int flags, mode_t mode);