CODING_STYLE: add section about using strlcat()

CODING_STYLE.md

@@ -672,3 +672,13 @@ string.
 Unless you have a valid reason to accept truncation you must check whether
 truncation has occurred, treat it as an error, and handle the error
 appropriately.
+
+#### Use `strlcat()` instead of `strncat()`
+
+When concatenating strings always use `strlcat()` instead of `strncat()`. The
+advantage of `strlcat()` is that it will always append a `\0` byte to the
+string.
+
+Unless you have a valid reason to accept truncation you must check whether
+truncation has occurred, treat it as an error, and handle the error
+appropriately.