removed tarval_P_void, every reference mode now has it's own NULL tarval

removed tarval_P_void, every reference mode now has it's own NULL tarval
which is created from the null_value given at initialization.

This also solves the problem the new_tarval_from_str() and new_tarval_from_long() returned different
tarvals for the same values.

Reference modes are handled like integer modes, however, convert is only supported for NULL pointer (which might be wrong if null_value != 0)

[r7465]