Add an option to ignore packets sourced from particular subnets.

It's implemented with a new set[subnet] option named ignore_checksums_nets.

If you populate this set with subnets, any packet with a src address within
that set of subnets will not have it's checksum validated.

src/Val.cc

@@ -1991,6 +1991,11 @@ ValPtr TableVal::FindOrDefault(const ValPtr& index)
 	return Default(index);
 	}
 
+bool TableVal::Contains(const IPAddr& addr)
+	{
+	return subnets ? subnets->Lookup(addr, true) : false;
+	}
+
 Val* TableVal::Lookup(Val* index, bool use_default_val)
 	{
 	static ValPtr last_default;