M2: live netfilter data path (pf_test per direction), pfattach bring-up, NET_LOCK reentrancy fix, static pfctl

- pfk_hooks: PRE→PF_IN, LOCAL_OUT/POST→PF_OUT v4+v6; replacement/buffer
  handling with NF_STOLEN + re-injection (documented divergence: re-entry)
- pfk_mod: pf_status_init/pfattach + pfi attach via pfk_if event callback
- pfk_kern: NET_LOCK holder reentrancy (fix deadlock when notifier fires
  under the write lock — observed as full-VM network hang)
- tools: static pfctl link for musl VMs

tools/include/sys/_null.h

@@ -0,0 +1,14 @@
+/* PFLINUX: <sys/_null.h> — OpenBSD's NULL guard header, required by the
+ * vendored <sys/queue.h> and <sys/tree.h>. */
+#ifndef _PFLINUX_SYS__NULL_H
+#define _PFLINUX_SYS__NULL_H
+
+#ifndef NULL
+#ifdef __cplusplus
+#define NULL	0
+#else
+#define NULL	((void *)0)
+#endif
+#endif
+
+#endif /* _PFLINUX_SYS__NULL_H */