Problem writing out landmask to history file

I noticed this on the cesm2_3 branch as exact restart tests fail. But, it's an issue that we could have noted on the cesm2_1 branch as well. PIO is failing because _FillValue isn't properly set.

This bit of PIO code is failing (pio_darray.c):

``` C
/* Check that if the user passed a fill value, it is correct. If
* use_fill is false, then find_var_fillvalue will not end up
* getting a fill value. */
if (fillvalue && vdesc->use_fill)
if (memcmp(fillvalue, vdesc->fillvalue, vdesc->pio_type_size))
return pio_err(ios, file, PIO_EINVAL, __FILE__, __LINE__);
```

I think this just means that we need to explicitly set missing_value and _FillValue in the definition of history and restart variables. So I think it's a straightforward fix.