Fortran: Add user defined error messages for UDTIO.

The defines IOMSG_LEN and MSGLEN were redundant so these are combined
into IOMSG_LEN as defined in io.h.

The remainder of the patch adds checks for when a user defined
derived type IO procedure sets the IOSTAT or IOMSG variables
independent of the librrary defined I/O messages.

	PR libfortran/105456

libgfortran/ChangeLog:

	* io/io.h (IOMSG_LEN): Moved to here.
	* io/list_read.c (MSGLEN): Removed MSGLEN.
	(convert_integer): Changed MSGLEN to IOMSG_LEN.
	(parse_repeat): Likewise.
	(read_logical): Likewise.
	(read_integer): Likewise.
	(read_character): Likewise.
	(parse_real): Likewise.
	(read_complex): Likewise.
	(read_real): Likewise.
	(check_type): Likewise.
	(list_formatted_read_scalar): Adjust to IOMSG_LEN.
	(nml_read_obj): Add user defined error message.
	* io/transfer.c (unformatted_read): Add user defined error
	message.
	(unformatted_write): Add user defined error message.
	(formatted_transfer_scalar_read): Add user defined error message.
	(formatted_transfer_scalar_write): Add user defined error message.
	* io/write.c (list_formatted_write_scalar): Add user defined error message.
	(nml_write_obj): Add user defined error message.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr105456-nmlr.f90: New test.
	* gfortran.dg/pr105456-nmlw.f90: New test.
	* gfortran.dg/pr105456-ruf.f90: New test.
	* gfortran.dg/pr105456-wf.f90: New test.
	* gfortran.dg/pr105456-wuf.f90: New test.