re PR libfortran/19052 (unit 0 not preconnected to standard error)

re PR libfortran/19052 (unit 0 not preconnected to standard error)

        PR libgfortran/19052
        * libgfortran.h (options_t): Add stderr_unit.
        * io/io.h (error_stream): Declare.
        * io/open.c (new_unit): Do not terminate abnormally if opening
        file preconnected to stdin, stdout, or stderr.
        * io/unit.c (init_units): Initialize stderr_unit.
        * io/unix.c (error_stream): New function.
        * runtime/environ.c (GFORTRAN_STDERR_UNIT): New environment variable.

From-SVN: r94090