Excellent!

This is rather nifty! Excellent work.
I have three questions though:

1. would it be hard to convert -m into -m DELIMITER to allow the usage of e.g. : instead of ',' as some locales unhelpfully use , as decimal separator?

2. would it be possible to also add a high resolution RTT field that exploits the fact that on the sender side we have higher timestamp resolution than 1ms for both the Originate and the Finished timestamps? (Reporting these with millisecond granularity is excellent, so all I want is essentially an addition RTT field) same for --icmp-echo... the high resolution data is already there so why not exploit it? Or would that require too much local state?

3. in --icmp-echo mode the -D, --print-timestamps Print UNIX timestamps for responses option seems to not work:

user@ubuntu:~/CODE/tsping/builddir$ sudo ./tsping -D -r 50  -s 0 9.9.9.9 51.75.49.241 145.239.2.3 51.91.81.112 79.137.19.131 62.109.121.1 --icmp-echo -D
Starting tsping 0.1 - pinging 6 targets
9.9.9.9         : [0], 9 ms

Start by reproducing the reported --icmp-echo and -D behavior using the command shown in the issue, then inspect the option handling and timestamp/RTT reporting paths. Clarify which of the delimiter, high-resolution RTT, and timestamp requests is in scope; completion requires an agreed change and verification of the relevant output.