Even though there is no need to manually release resources for NetPlain
and NetTLS, for other networkers - for example lonet - releasing
resources is needed. In general programs cannot always use lonet
explicitly, as there might be a factory function that creates a
Networker according to given parameters.

-> Adjust Networker interface to include general Close.
-> Adjust NetPlain to follow added "interrupt-on-close" semantic for uniformity.