clock time to uptime because wall clock time may go backwards.
This is a change in the API which will impact SNMP agents who are using
ifi_epoch to set RFC2233's ifCounterDiscontinuityTime. None are know to
exist today. This will not impact applications that are using the
<index, epoch> tuple to verify interface uniqueness except that it
eliminates a race which could lead to a false assumption of uniqueness.
Because this is a behavior change, bump __FreeBSD_version.
Discussed with: re (jhb, scottl)
MFC after: 3 days
Pointed out by: pkh (way back at EuroBSDCon)
Pointy hat: brooks
bc9d299133