nixos/znc-service: don't use types.string (it's deprecated)

Apart from s/types.string/types.str/ (or types.lines where appropriate):

* port is changed from string to int.

* extraFlags is changed from types.string (with unfortunate merge
  semantics) into a list of strings. A list of strings merge better:
  one space is added between elements.
Merge pull request #3262 from bjornfor/znc-module-types

nixos/znc-service: don't use types.string (it's deprecated)