freedns: fix domain splitting for multi-part domains

[frebib]

It seems that the way domains were split before was using some unnecessarily complex maths which was broken for domains >3 parts (e.g _acme-challenge.test.domain.com)

I assume what the correct behaviour should be is the following:

sub_domain='_acme-challenge'
top_domain='everything.after.the.first.dot.com'

This PR fixes for domains >3 parts and still works for simple sub.domain.com domains

[martgras]

Does that also work if I have separate zone for a subdomain.
Let's say I have example.com. I need hostnames like www.devorg.example.com mail.devorg.example.com
www.sales.example.com etc.
Of course I can create the records in the example.com (like www.devorg) but the devorg team wants to manage entries in their subdomain . For security reasons I don't want them to have control over anything else than devorg.example.com. The solution is creating a new DNS zone for devorg.example.com
This becomes more important with the DNS alias mode where I can use a subdomain for the validation records without the need to grant access to the complete example.com zone

[frebib]

I'm not entirely sure, are you able to test it?
Did it work before this patch?

[martgras]

I never used freedns.
Most implenations start splitting at the first dot. Then check if the domain is in the list of domains from your provider.
If not split the domain again until you have a match.

[frebib]

This addresses the issue in #1086.

As for checking each part in turn, that will need some testing.
I'd suggest a separate issue should be opened to track that? It looks like it would be reasonably significant rewrite of the code in this function