gnrc_netif: introduce `is_6lo()` as alias to `is_6ln()`
gnrc_netif: introduce `is_6lo()` as alias to `is_6ln()`

Preparation step to introduce a semantic difference between an
interface being a 6Lo interface and a 6LN according to RFC 6775 [[1]]
(i.e. performs Neighbor Discovery as defined there).

[1]: https://tools.ietf.org/html/rfc6775#section-2

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