gfortran.dg/read_dir.f90: Make PASS on Windows

gfortran.dg/read_dir.f90: Make PASS on Windows

On non-Cygwin Windows, use '.' and expect the documented fail when opening
a directory (EACCESS).  As gfortran does not set __WIN32__ this check is
done on the C side. (On __CYGWIN__, __WIN32__ is not set - but to make it
clear, !__CYGWIN__ is used in #if.)

On non-Windows, replace the 'call system' shell call by the POSIX functions
stat/mkdir/rmdir for better compatibility, especially on embedded systems;
additionally add some more checks. In particular, confirm that 'close' with
status='delete' indeed deleted the directory.

gcc/testsuite/ChangeLog:

	* gfortran.dg/read_dir-aux.c: New; provides my_mkdir, my_rmdir,
	my_verify_not_exists and expect_open_to_fail.
	* gfortran.dg/read_dir.f90: Call those; expect that opening a
	directory fails on Windows.