- add `netif_get_ipv6()` to query IPv6 (as a slim convenience wrapper
for `netif_get_opt()`)
- add `netifs_get_ipv6()` to query IPv6 addresses of all netifs
- add `netif_print_ipv6()` to print the IPv6 address(es) of a single
netif
- add `netifs_print_ipv6()` to print the IPv6 address(es) of all netifs
For consistency with `gnrc_netif_iter()`, add the `const` qualifier to the
parameter of `netif_iter()`. This allows calling it on `const` pointers without
having to cast `netif_t *`, which (apart of dropping the `const` qualifier)
disables a lot of type safety checks offered by the compiler.
