Support DHCP relay on interfaces with zero MAC address

The dhcrelay daemon is designed to work on Ethernet-alike interfaces with MAC addresses. However, some interfaces on Linux don't use L2 header at all, such as cellmodem's wwanX interfaces, if the IPSec kernel driver doesn't forge a MAC address, the ipsecX interfaces built upon such wwanX interfaces won't have L2 addresses, resulting in the dhcrelay daemon unable to use them.

I will try to propose a patchset to support such interfaces.