The function to infer the link-layer address length from the length of
a S/TLLAO is very dependent on the IPv6 over X specification and thus
should be grouped with the other IP over X functions.

This way the IID generation gets much simplified and code duplication
is avoided since it removes GNRC's requirement for NETOPT_IPV6_IID.

the names for iid_from_addr and iid_to_addr functions in the
else-block were switched around, so this fixes that

They are required to make GNRC's 6Lo IPHC implementation ready for
hardware addresses other than IEEE 802.15.4's.

I applied the following terminology and changed the wording in the doc
accordingly:

* must not: If the parameter is of the value it *must not* be it either
  hits an assert or crashes the system.
* may not: The value can be that value, but the function will return an
  error.

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'
```