Make substructures for IPv4 and IPv6 specific context information

The context structure contains a batch of fields specific to IPv4 and to
IPv6 connectivity.  Split those out into a sub-structure.

This allows the conf_ip4() and conf_ip6() functions, which take the
entire context but touch very little of it, to be given more specific
parameters, making it clearer what it affects without stepping through the
code.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -278,8 +278,8 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 		if (bind_addr) {
 			addr6.sin6_addr = *(struct in6_addr *)bind_addr;
 
-			if (!memcmp(bind_addr, &c->addr6_ll,
-			    sizeof(c->addr6_ll)))
+			if (!memcmp(bind_addr, &c->ip6.addr_ll,
+			    sizeof(c->ip6.addr_ll)))
 				addr6.sin6_scope_id = c->ifi6;
 		} else {
 			addr6.sin6_addr = in6addr_any;