Are namelist defaults set correctly, specifically related to megan settings for B1850 case?

Looking at a run Cecile did in [CESM_dev#280](https://github.com/NCAR/cesm_dev/issues/249)

She created the case like this:
```
./create_newcase --compset B1850C_LTso --res ne30pg3_t232_wg37 --case /glade/campaign/cesm/cesmdata/cseg/runs/cesm2_0/b.e30_alpha08a.B1850C_LTso.ne30_t232_wgx3.280 --run-unsupported --project 93300722
```

But CLM_BLDNML_OPTS has a `--no-megan` flag, which doesn't seem correct (or is at least confusing if this is set elsewhere)?
```

```

@slevis-lmwg also noticed that in an I1850 case he sees `megan_use_gamma_sm = .false. `, but this does not show up in the lnd_in file on the B-case. It seems like it should?

@ekluzek can you help clarify here?