Do not fail if using fping6 with -6 flag

[Roman Plessl]

My Ubuntu 20.04 smokeping setup was configured with binary name fping6, and smokeping 2.7.3-2 which always uses -6.

Using fping 4.2-1 on Ubuntu 20.04 the fping6-probes from smokeping stopped working.

Using fping 4.0-6 on Ubuntu 18.04 the fping6-probe works as expected.

After some digging, it turns out fping6 and -6 is broken, the program will exit with
'fping6: can't specify both -4 and -6'

root@probe:~# fping6 -6 ipv6.google.com
fping6: can't specify both -4 and -6

fping is working fine, as expected

root@probe:~# fping -6 ipv6.google.com
ipv6.google.com is alive

---

root@probe:~# lsb_release -rd
Description: Ubuntu 20.04 LTS
Release: 20.04

root@probe:~# apt-cache policy fping
fping:
  Installed: 4.2-1
  Candidate: 4.2-1
  Version table:
 *** 4.2-1 500
        500 http://ch.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status

[Roman Plessl (roman-plessl)]

Upstream the Maintainer has already included a fix:

https://github.com/schweikert/fping/pull/149

[Axel Beckert (xtaran)]

Uploaded the fix to Debian Unstable. Should reach Ubuntu Groovy via automatic sync from Debian Unstable.

[Launchpad Janitor (janitor)]

This bug was fixed in the package fping - 4.2-3

---------------
fping (4.2-3) unstable; urgency=medium

  * Cherry-pick upstream patch "Do not fail if using fping6 with -6
    flag". (Closes: #955436, LP: #1875242)
  * Bump debhelper compatibility level to 13 via build dependency.

 -- Axel Beckert <email address hidden> Mon, 27 Apr 2020 15:37:16 +0200