doc: add API helper files

These files should be a very quick reference to the Xtables APIs of
previous Linux kernel versions and Xtables-addons. Their contents have
been reformatted so as to be usable with diff -u.

doc/api/2.6.24.c

@@ -0,0 +1,59 @@
+match:
+
+	/* true/false */
+	bool
+	(*match)(
+		const struct sk_buff *skb,
+		const struct net_device *in,
+		const struct net_device *out,
+		const struct xt_match *match,
+		const void *matchinfo,
+		int offset,
+		unsigned int protoff,
+		bool *hotdrop,
+	);
+
+	/* true/false */
+	bool
+	(*checkentry)(
+		const char *tablename,
+		const void *ip,
+		const struct xt_match *match,
+		void *matchinfo,
+		unsigned int hook_mask,
+	);
+
+	void
+	(*destroy)(
+		const struct xt_match *match,
+		void *matchinfo,
+	);
+
+target:
+
+	/* verdict */
+	unsigned int
+	(*target)(
+		struct sk_buff *skb,
+		const struct net_device *in,
+		const struct net_device *out,
+		unsigned int hooknum,
+		const struct xt_target *target,
+		const void *targinfo,
+	);
+
+	/* true/false */
+	bool
+	(*checkentry)(
+		const char *tablename,
+		const void *entry,
+		const struct xt_target *target,
+		void *targinfo,
+		unsigned int hook_mask,
+	);
+
+	void
+	(*destroy)(
+		const struct xt_target *target,
+		void *targinfo,
+	);