Replace strncat with _strlcat

Replace strncat with _strlcat

Using strncat is hard. The function expects you to know how many bytes
you can write in the buffer instead if figuring out itself.

Introduce a _strlcat call which correctly handles the buffer length
handling and guarantees the data is null terminated after the operation.

Even though the code here was handling the buffer length correctly,
the code is much simpler if _strlcat is used instead.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>