net.tls_padding() can now take a boolean in addition to a numeric
value.  true means "use sensible default padding policy", false means
"never pad".

In the struct kr_context, we change tls_padding from a uint32_t to an
int32_t so that we can explicitly represent the default value (-1).
This should be a safe ABI/API change, since no one had ever set a
padding > 4096 anyway.

This depends on libknot having adopted the changes from
2dd9f406e333a1cedfa2380ffad57913cecf8efb which is first included in
2.4.3.  In the event that knot-resolver is compiled against an older
version of libknot, it just uses a baked-in default of 468 as before.

See https://gitlab.labs.nic.cz/knot/resolver/merge_requests/249 for
further discussion.