More fixes to String porting
More fixes to String porting

nastrings is often specified separately from truestrings and falsestrings,
yet ParseOptions required the vector types to be the same. Also clean
a few details related to the port.

JuliaPackageMirrors/DataFrames.jl,scls19fr/DataFrames.jl,JuliaStats/DataFrames.jl,campanja-forks/DataFrames.jl