Add support for NAT64 prefix sizes other than /96 (RFC6052)

[Leseratte10]

This PR fixes a couple small bugs I've noticed:

• The IPv4 subnet in the config file - which previously had to be entered like 192.168.0.0/255.255.0.0 can now also just be entered as 192.168.0.0/16.
• Previously, the code assumed that the NAT64 prefix size was always /96 and that the IPv4 address was always stored in the last four bytes of the IPv6 address. With RFC6052, other prefix sizes larger than /96 can also be used, which have the IPv4 address stored at another position in the IPv6 address. The remaining unused bits at the end are the so-called suffix which can now be set using the config option nat64_suffix, if needed. If not set, these bits are set to 0.

Right now I haven't yet been able to fully test this, so I'm making it a draft PR in case you already want to look at the code. I'm going to try to do a bunch of testing in the next couple days to make sure that's all working as intended.

[andrewshadura]

You probably wanted to rebase instead :)

[Leseratte10]

Yeah, I did. Stupid git. Fixed.

[Leseratte10]

Okay, I think this PR should now be somewhat ready for review as well if you want to take a look.

Once my other PR (#8) is merged I'm going to add a test or two for this PR as well (so the other PR should probably be reviewed and merged first), I don't want to do that now so I don't get merge conflicts again that I need to fix.

[andrewshadura]

Thanks!

I saw you added a test based on Python — since the main implementation of Meson is written in Python (and I want to add Meson build files), yeah, maybe it’s reasonable after all to require Python for tests. Then we probably don’t need to try and find dig, netcat, telnet and other networking tools 🙂

[Leseratte10]

The python test was in the other PR (netlink), but yeah, that's what I thought. Python is not that uncommon, and I don't think writing a netlink test in Bash or plain SH is going to be easy.