generate icmp checksum

[Florian Weimer]

* Peng Yu via Libc-help:

> Hi,
>
> https://github.com/bminor/glibc/blob/21c3f4b5368686ade28d90d8c7d79c4c95c72c1b/sysdeps/gnu/netinet/ip_icmp.h
>
> I see there is a checksum field in the above file. Is there a function
> that can generate the checksum? Thanks.

I do not think we have an implementation of the Internet checksum in
glibc, but it's a large library, so it's difficult to be sure.

Florian