Handle tun/tap device returning EPERM or EBUSY.

Often when tun/tap is used any errors during setup will be confuse tinc
and it will then assume it is an Ethertap device. Try to avoid this by
checking errno after a failed TUNSETIFF; if it's EPERM or EBUSY then
we can be sure it was not an Ethertap device, and we should report an
error instead.

Closes #157 on GitHub.