PR libfortran/20068
	PR libfortran/20125
	PR libfortran/20156
	PR libfortran/20471

	* io/backspace.c (unformatted_backspace): Fix error in arithmetic.
	(st_backspace): When in WRITING mode, we flush and falling back
	into READING mode. In all cases, correctly position the stream.

	* gfortran.dg/backspace.f: New test.

Co-Authored-By: Francois-Xavier Coudert <coudert@clipper.ens.fr>

From-SVN: r97478