Fix for issue #3326

[advait deshmukh <adv2823deshmukh () gmail com>]

Issue link <https://github.com/nmap/nmap/issues/3326>
Pull request link <https://github.com/nmap/nmap/pull/3337>
I read the source code and, from what I understood, the current output
appears to be intentional. The ipv4 value being shown seems to refer to the
next header, i.e., the protocol of the packet encapsulated within the outer
packet, which in this case is IPv6.
Since the user has explicitly specified -6 in the command, it seems
reasonable to assume that the outgoing packets are expected to be in IPv6
format rather than IPv4.
That said, I understand how this could lead to confusion. To improve
clarity, I have added a fix in the else statement at line 3127
<https://github.com/Adv2924/nmap/blob/6dac43b5d892e5566f4d1d911573403e9f86bb96/libnetutil/netutil.cc#L3127>
in libnetutil/netutil.cc, which checks whether hdr.version is set to 6 and
updates the logging accordingly.
While going through the code I also noticed that nmap tries to parse the
packet it recognizes (it was ICMP in this case). So I wanted to ask if it
will be helpful if we can parse other packets too (whichever are mentioned
in the massive switch case in nexthdrtoa function in netutil.cc)?
PS: I am completely new to open source contributing and this is my first
pull request, so please don't mind me asking stupid questions.

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