Improve MAC address uniqueness on if_epair(4).

As reported in PR184149, it can happen that epair devices can have the same
MAC address.
This solution is based on a 32-bit hash, obtained combining the if_index of
the a interface and the hostid.
If the hostid is zero, a random number is used.

PR: 184149
Reviewed by: wollman, eugen
Approved by: cognet
Differential Revision: https://reviews.freebsd.org/D15329