Query the Linux device for its MAC address.

Query the Linux device for its MAC address.

On Linux, tinc doesn't know the MAC address of the TAP device until the
first read. This means that if no packets are sent through the
interface, tinc won't be able to figure out which MAC address to tag
incoming packets with. As a result, it is impossible to receive any
packet until at least one packet has been sent.

When IPv6 is disabled Linux does not spontanously send any packets
when the interface comes up. At first users wonder why the node is not
responding to ICMP pings, and then as soon as at least one packet is
sent through the interface, pings mysteriously start working, resulting
in user confusion.

This change fixes that problem by making sure tinc is aware of the
device's MAC address even before the first packet is sent.