refactor(splits): Minor cleanups to periph invocation

* Add strlcpy from public domain version.
* Leverage strlcpy to detect truncation of behavior dev strs, and log.
* Use `offsetof` for cleaner detection on peripheral side.

app/include/zmk/stdlib.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <stdlib.h> /* for size_t */
+
+/*
+ * ANSI C version of strlcpy
+ * Based on the NetBSD strlcpy man page.
+ *
+ * Nathan Myers <ncm-nospam@cantrip.org>, 2003/06/03
+ * Placed in the public domain.
+ */
+
+size_t strlcpy(char *dst, const char *src, size_t size);