Fix FS#25002 (add net-tools dep for now), FS#23452 (fix path, add
optdepend) and FS#25095 (add supplied patch).
Please sign off.
I also want to mention that I don't use vpnc anymore, and haven't done
so for years. I don't want to keep maintaining it, so if there are
takers among the devs, please step up.