Fixes the following error with GCC 7
main.c: In function 'test_netif_get_name':
main.c:1054:23: error: 'sprintf' may write a terminating nul past the end of the destination [-Werror=format-overflow=]
sprintf(exp_name, "if%d", (int)netif);
^~~~~~
main.c:1054:5: note: 'sprintf' output between 4 and 9 bytes into a destination of size 8
sprintf(exp_name, "if%d", (int)netif);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `test_ipv6_group_join__ENOMEM()` tests joining multicast groups,
so we need to iterate `GNRC_NETIF_IPV6_GROUPS_NUMOF` times to fill up
all addresses, not `GNRC_NETIF_IPV6_ADDRS_NUMOF`.
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'
```
