nping echo server doesn't fully support IPv6

[marek22k]

Describe the bug
Connecting with IPv6 is successful.

To Reproduce

$ sudo ./nping/nping --echo-server public -v4 --once -6

Starting Nping 0.7.98SVN ( https://nmap.org/nping ) at 2025-10-26 14:13 UTC
Packet capture will be performed using network interface dnet.
Waiting for connections...
Server bound to :::9929

$ sudo ./nping/nping --echo-client "public" ::1 -v4 -6

Starting Nping 0.7.98SVN ( https://nmap.org/nping ) at 2025-10-26 14:13 UTC
Connection failed.
 
Max rtt: N/A | Min rtt: N/A | Avg rtt: N/A
Raw packets sent: 0 (0B) | Rcvd: 0 (0B) | Lost: 0 (0.00%)| Echoed: 0 (0B) 
Tx time: 0.00000s | Tx bytes/s: 0.00 | Tx pkts/s: 0.00
Rx time: 0.00000s | Rx bytes/s: 0.00 | Rx pkts/s: 0.00
Nping done: 1 IP address pinged in 0.00 seconds

$ sudo ./nping/nping --echo-client "public" echo.nmap.org -v4 -6

Starting Nping 0.7.98SVN ( https://nmap.org/nping ) at 2025-10-26 14:14 UTC
Connection failed.
 
Max rtt: N/A | Min rtt: N/A | Avg rtt: N/A
Raw packets sent: 0 (0B) | Rcvd: 0 (0B) | Lost: 0 (0.00%)| Echoed: 0 (0B) 
Tx time: 0.00000s | Tx bytes/s: 0.00 | Tx pkts/s: 0.00
Rx time: 0.00000s | Rx bytes/s: 0.00 | Rx pkts/s: 0.00
Nping done: 1 IP address pinged in 0.00 seconds

$ git log -n1
commit fcf6f0cb19e829720faa9f5ace506a582253104b (HEAD -> master, origin/master, origin/HEAD)
Author: nnposter <nnposter@e0a8ed71-7df4-0310-8962-fdc924857419>
Date:   Wed Oct 8 23:08:04 2025 +0000

    Use simpler/cheaper portrule

Expected behavior
Connection works.

Version info (please complete the following information):

• OS: Arch Linux

$ ./nping/nping --version

Nping version 0.7.98SVN ( https://nmap.org/nping )
$ ./nmap --version
Nmap version 7.98SVN ( https://nmap.org )
Platform: x86_64-unknown-linux-gnu
Compiled with: liblua-5.4.8 openssl-3.6.0 libssh2-1.11.1 libz-1.3.1 libpcre2-10.47 libpcap-1.10.5 nmap-libdnet-1.18.0 ipv6
Compiled without:
Available nsock engines: epoll poll select

Additional context
Connection via IPv4 still results in #2547 (comment).