net_help: remove printArrayRange()

The od module does the same, much less specialized, much more
sophisticated.
net_help: remove IPV6_CMP_ADDR macro

Used nowhere; alternative: ipv6_addr_is_equal(), since other use-cases
(is an IPv6 address smaller than the other) are not applicable anyway.
net_help: prefix csum properly and add documentation
Merge pull request #2019 from authmillenon/cleanup-net_help

net_help: Cleanup