gnrc_netif: rename IPv6 address "setters"

This renames the following functions

* `gnrc_netif_ipv6_addr_add()`
* `gnrc_netif_ipv6_addr_remove()`
* `gnrc_netif_ipv6_group_join()`
* `gnrc_netif_ipv6_group_leave()`

by appending the suffix `_internal`.

\## Reasoning

I'd like to provide a helper function for the *public* equivalent using
`gnrc_netapi_set()`, and those names are to nice to not be taken for
those.

\## Procedure
I used a combination of `git grep` and `sed` to do this and fixed the
alignment in the result of some cases by hand.

```sh
git grep --name-only "\<gnrc_netif_ipv6_\(addr\|group\)_\(add\|remove\|join\|leave\)\>" | \
        xargs sed -i 's/\<gnrc_netif_ipv6_\(addr\|group\)_\(add\|remove\|join\|leave\)/\0_internal/g'
```