CODING_STYLE: add section about using strlcpy()

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

CODING_STYLE.md

@@ -662,3 +662,13 @@ rules to use them:
   #endif
   };
   ```
+
+#### Use `strlcpy()` instead of `strncpy()`
+
+When copying strings always use `strlcpy()` instead of `strncpy()`. The
+advantage of `strlcpy()` 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.