treewide: Make a bunch of pointer variables pointers to const

Sufficiently recent cppcheck (I'm using 2.13.0) seems to have added another
warning for pointer variables which could be pointer to const but aren't.
Use this to make a bunch of variables const pointers where they previously
weren't for no particular reason.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.c

@@ -48,8 +48,8 @@
 char *ipv6_l4hdr(const struct pool *p, int idx, size_t offset, uint8_t *proto,
 		 size_t *dlen)
 {
-	struct ipv6_opt_hdr *o;
-	struct ipv6hdr *ip6h;
+	const struct ipv6_opt_hdr *o;
+	const struct ipv6hdr *ip6h;
 	char *base;
 	int hdrlen;
 	uint8_t nh;