Use asynchronous load and dump IO in named-checkzone, dnssec-signzone and possibly others
Currently, in main, sending the SIGINT does nothing if the zone is loading or dumping because we run that on the main thread.
We need to switch to use asynchronous IO and then use dns_loadctx_cancel()
and dns_dumpctx_cancel()
when SIGINT is received, so we can properly interrupt the zone loading and dumping.
This applies to all utilities that can load and dump zones, notably named-checkzone
and dnssec-signzone
.