net-mgmt/telegraf: Add ping6 input

[DasSkelett]

Background

Many months ago I discovered that Telegraf's ping input plugin doesn't work for IPv6 on OPNsense (and FreeBSD in general).
I fixed the general FreeBSD problems in this pull request for Telegraf, which is included since release 1.15.3.
OPNsense also needs some changes to make use of the fixed Telegraf.

Changes

Added another checkbox to enable a second instance of the ping input for IPv6, and a list box where one can specify the IPv6 hosts, to the GUI.
This second instance has binary = "ping6" set in telegraf.conf.
Both the IPv4 and the IPv6 instances also explicitly set method = "exec", since there's a chance that the ping input plugin will move to method = "native" in the future, which wouldn't work without changes (if at all?) on FreeBSD, because it needs raw ICMP sockets:

This plugin has two main methods of operation: exec and native. The recommended method is native, which has greater system compatibility and performance. However, for backwards compatibility the exec method is the default.
When using method = "native" a ping is sent and the results are reported in native Go by the Telegraf process, eliminating the need to execute the system ping command.
When using method = "native", Telegraf will attempt to use privileged raw ICMP sockets.

More info can be found in the README for the ping input plugin.

Testing

I've tested these changes on my main OPNsense router, and they seem to work fine with a replaced telegraf binary.

[DasSkelett]

With Telegraf 1.15.3 in the ports now, this pull request would be ready to go.

[mimugmail]

I installed the branch and it works fine, thanks! :)
Sorry for the delay, this is what happens when you also read github notification on a mobile, totally forgot this one, sorry.

[fichtner]

Merged, thanks a lot ❤️