Return HTTP status code for small/malformed requests

Return HTTP status code for small/malformed requests

This commit makes BIND return HTTP status codes for malformed or too
small requests.

DNS request processing code would ignore such requests. Such an
approach works well for other DNS transport but does not make much
sense for HTTP, not allowing it to complete the request/response
sequence.

Suppose execution has reached the point where DNS message handling
code has been called. In that case, it means that the HTTP request has
been successfully processed, and, thus, we are expected to respond to
it either with a message containing some DNS payload or at least to
return an error status code. This commit ensures that BIND behaves
this way.