first_boot_pending() is true while /firstboot exists on FreeBSD. A FreeBSD
cloud image upgrades its base system on its first boot, starts sshd only
after that and restarts right away, and /etc/rc removes /firstboot just
before that restart. netOrk waits for this before it sets up a new VM
(NetOrk/netork#795).
OpenBSD has no such marker to ask yet, so it returns False there.