Support namecoin standard 'map' for subdomain name

[rllola]

See #1843

[HelloZeroNet]

Thanks, please use other variable name instead of map (eg. data_map) as it's a built-in function in python and changing it could have some side-effects and rename newValue to new_value to make the variable names consistent within the function.

[rllola]

Done.

I have also added a new case I forgot https://github.com/HelloZeroNet/ZeroNet/pull/1856/files#diff-ccde09c3a724251c612f29d91a7751d7R47

It is when you only have :

{
   "zeronet":"19rXKeKptSdQ9qt7omwN82smehzTuuq6S9"
}

I choose to reformat the namecoin standard in the zeronet one and recall the function so I don't have to modify anything in the part where you save it. I hope this is fine. I prefer to make a minimal modification.