Change the default of net.link.epair.ether_gen_addr 0 -> 1.

Now if_epair(4) uses ether_gen_addr(9) to generate a stable MAC.
This feature was committed in 590493c1419092.

Approved by: kp
MFC after: never
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D51157