feat: say whether a host is still in its first boot

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.

README.md

@@ -38,6 +38,7 @@ over SSH, built on [napalm-device-types](https://git.netork.io/NAPALM/napalm-dev
 | `get_services`, `manage_service` | ✓ | ✓ | `service -e` + `service … status` / `service … <action>`; `rcctl ls on` + `rcctl check` / `rcctl <action>` |
 | `run_device_action("fix_snmp")`, `get_snmp_config` | ✓ | ✓ | net-snmp from packages: `/usr/local/etc/snmp/snmpd.conf` + `service snmpd`; `/etc/snmp/snmpd.conf` + `rcctl … netsnmpd` |
 | `get_host_status` | ✓ | unknown | napalm-device-types' host status: on FreeBSD `freebsd-version -k` vs `-r` (4.1+) |
+| `first_boot_pending` | ✓ | always `False` | whether `/firstboot` still exists |
 
 `get_listening_sockets` has the shape of napalm-device-types'
 `ListeningSocketsMixin` (whose `ss`/cgroup reading is Linux's) and its rule:
@@ -68,6 +69,11 @@ database pkg cannot read (only pkg's "not installed" means no packages).
 (`freebsd-version -k`) differs from the running one (`-r`) as needing a reboot.
 OpenBSD has no such reading yet, so it stays unknown there.
 
+**First boot.** A FreeBSD cloud image upgrades its base system on its first boot,
+starts sshd only after that and restarts right away. `/etc/rc` removes `/firstboot`
+just before the restart, so `first_boot_pending()` is true until then; netOrk waits
+for it before it sets up a new VM. OpenBSD has no such marker to ask yet.
+
 **Services** are the enabled ones. FreeBSD reads their status as root, as
 root-only pidfiles hide a daemon from anyone else, and without root when sudo
 refuses; OpenBSD's `rcctl check` needs no root. Actions run as root.