net_help: Cleanup

In addition to #1984 this cleans up the `net_help` module.

* removes `printArrayRange()`. Alternative: the `od` module
* removes `IPV6_CMP_ADDR()`. Alternative: `ipv6_addr_is_equal()`/`ipv6_addr_equal()` (as of #2003). Other comparisons between IPv6 addresses would be odd anyway
* renames `csum()` according to Coding Conventions to `net_help_csum()`