Allow opening file on multiple units

Allow opening file on multiple units

As of Fortran 2018 it's allowed to open the same file on multiple
units.

libgfortran/ChangeLog:

2019-05-15  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/90461
        * io/open.c (new_unit): Don't check if the file is already open
	for F2018.

testsuite/ChangeLog:

2019-05-15  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/90461
        * gfortran.dg/open_errors_2.f90: Add -std=f2008, adjust line number.
	* gfortran.dg/open_errors_3.f90: New test.

From-SVN: r271260