Set networking.firewall.allowPing = true by default.

This patch fixes https://github.com/NixOS/nixpkgs/issues/12927.

It would be great to configure good rate-limiting defaults for this via
/proc/sys/net/ipv4/icmp_ratelimit and /proc/sys/net/ipv6/icmp/ratelimit,
too, but I didn't since I don't know what a "good default" would be.