mdns: apply network-probing fix from mdns_listen to mdns_serve

[Blaxar]

Long story short: alloca was invoked in the main (not returning) loop of mdns_serve, so as long as we don't get out of said loop: nothing gets unwinned (stack memory by default, heap memory if built if -fsplit-stack, at least with gcc on Linux-based systems) and memory is slowly munched away over the lifetime of whatever program relies on this.

As the commit message already hints: there was a similar fix applied to mdns_listen which just consists in having alloca being called into its own function (which will be unwinned for sure), but this was still missing for mdns_serve: #15