net/echoping: readd the port

echoping is non-default optional but very useful dependency
of the SmokePing (net-mgmt/smokeping). I use the software and
will maintain the port.

While here, make it possible to build echoping with GNU dns/libidn2
instead of dns/libidn. For now, still use libidn to preserve POLA.

Default build not changed comparing with one before removal,
so PORTREVISION not bumped.