PR libgfortran/83225
* io/io.h (is_internal_unit): Use the unit_is_internal bit.
* io/transfer.c (data_transfer_init): Set the bit to true for
internal umits. Use that bit for checks for internal unit
initializations.
* io/unit.c (insert_unit): As a precaution, set the
internal_unit_kind to zero when a unit structure is first created.