ping: 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' symbol.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC after: 2 weeks
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21288