Check if /etc/os-release is up-to-date on FreeBSD

Summary:
Normally, /etc/os-release should be regenerated at boot time.
If we don't boot after a userland update, the file won't be updated either.

Raise a warning if the file is not up-to-date, a critical error if it doesn't exist.

Ref T2007.

Test Plan: Tested on WindRiver

Reviewers: DorianWinty, aceppaluni

Reviewed By: DorianWinty, aceppaluni

Maniphest Tasks: T2007

Differential Revision: https://devcentral.nasqueron.org/D3416