Use default padding policy for responses.

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 in:

https://gitlab.labs.nic.cz/labs/knot/merge_requests/692

Once that's done, if there's a release, then this changeset needs to also update kresd's versioned dependency on libknot.

daemon/README.rst

@@ -604,10 +604,13 @@ For when listening on ``localhost`` just doesn't cut it.
 
 .. function:: net.tls_padding([padding])
 
-   Get/set EDNS(0) padding.  If set to value >= 2 it will pad the answers
-   to nearest *padding* boundary, e.g. if set to `64`, the answer will
-   have size of multiplies of 64 (64, 128, 192, ...).  Setting padding to
-   value < 2 will disable it.
+   Get/set EDNS(0) padding of answers to queries that arrive over TLS
+   transport.  If set to `true` (the default), it will use a sensible
+   default padding scheme, as implemented by libknot if available at
+   compile time.  If set to a numeric value >= 2 it will pad the
+   answers to nearest *padding* boundary, e.g. if set to `64`, the
+   answer will have size of a multiple of 64 (64, 128, 192, ...).  If
+   set to `false` (or a number < 2), it will disable padding entirely.
 
 .. function:: net.outgoing_v4([string address])