nds3: allow to add CNAME records without A records and multiple host entries

now it is possible to add CNAME only records, e.g.
{"host": [{"abc.github.io.": "", "alias": ["www"]}]}
results in
"www IN CNAME luschentreff.github.io."

and we can define multiple host entries by appending "_", e.g.

{"host": [{"@": "a.b.c"}, {"@_": "c.d.e"}]}
results in
"@ IN A a.b.c
 @ IN A c.d.e"