Fix -Wshadow warnings and clean up xt_sctp.h

Note: xt_sctp.h is still not merged upstream in the kernel as of
this commit. But a refactoring was really needed.
diff --git a/iptables-xml.c b/iptables-xml.c
index 404d020..94f2e39 100644
--- a/iptables-xml.c
+++ b/iptables-xml.c
@@ -99,16 +99,15 @@
 char curTable[IPT_TABLE_MAXNAMELEN + 1];
 char curChain[IPT_CHAIN_MAXNAMELEN + 1];
 
-typedef struct chain
-{
+struct chain {
 	char *chain;
 	char *policy;
 	struct ipt_counters count;
 	int created;
-} chain;
+};
 
 #define maxChains 10240		/* max chains per table */
-static chain chains[maxChains];
+static struct chain chains[maxChains];
 static int nextChain = 0;
 
 /* funCtion adding one argument to newargv, updating newargc