This commit ensures that isc_nm_cancelread() is not called from within
dig code for HTTP sockets, as these lack its implementation.
It does not have much sense to have it due to transactional nature of
HTTP.
Every HTTP request-response pair is represented by a virtual socket,
where read callback is called only when full DNS message is received
or when an error code is being passed there. That is, there is nothing
to cancel at the time of the call.
90c52ca12b