Cygwin: open(2): reset sparseness on O_TRUNCated files

Cygwin: open(2): reset sparseness on O_TRUNCated files

open(2) implements O_TRUNC by just reducing the size of the file
to 0, to make sure EAs stay available.

Turns out, file sparseness is not removed this way either, so add
code to do just that.

Fixes: 603ef545bdbd ("* fhandler.cc (fhandler_base::open): Never open files with  FILE_OVERWITE/FILE_OVERWRITE_IF.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>