Move in_cksum() to a separate source file
This is a preparation step for adding ATF tests of in_cksum(), which
has been modified to operate on unaligned data. ping.o cannot be
linked to the test executable because both of them contain 'main'
Submitted by: Ján Sučan <firstname.lastname@example.org>
Sponsored by: Google, inc. (Google Summer of Code 2019)