Patch: Zone Fetcher processes updated zonelist.xml

The zone_fetcher utility does not yet support changes to the zonelist.xml file. The attached patch makes it able to do so, if sent a SIGUSR1.

The SIGUSR1 signal would normally be sent from the signer, after it has processed an "update" or "update XXX" command. That has also been added in this patch. The rest is up to the KASP Enforcer, which already invokes update on the signer.

To be able to handle very long lists of zones, recursive code for freeing zonelists has also been replaced with a linked-list alternative.