Cygwin: dirname: fix handling of leading slashes

Cygwin: dirname: fix handling of leading slashes

Per https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/xbd_chap04.html:

  "A pathname that begins with two successive slashes may be interpreted
   in an implementation-defined manner, although more than two leading
   slashes shall be treated as a single slash."

So more than 2 leading slashes are supposed to be folded into one,
which our dirname neglected.  Fix that.

Fixes: 24e8fc6872a3b ("* cygwin.din (basename): Export.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>