hostinfo: add FreeBSD support.

Add specific handling for common appliances based on FreeBSD:
- pfSense
    HostInfo: {"OS":"freebsd","OSVersion":"pfSense 2.5.2-RELEASE; version=12.2-STABLE"
- OPNsense
    HostInfo: {"OS":"freebsd","OSVersion":"OPNsense 21.7.1 (amd64/OpenSSL); version=12.1-RELEASE-p19-HBSD"
- TrueNAS
    HostInfo: {"OS":"freebsd","OSVersion":"TrueNAS-12.0-U5.1 (6c639bd48a); version=12.2-RELEASE-p9"
- FreeNAS
    HostInfo: {"OS":"freebsd","OSVersion":"FreeNAS-11.3-U5 (2e4ded5a0a); version=11.3-RELEASE-p14",

- regular FreeBSD
    HostInfo: {"OS":"freebsd","OSVersion":"FreeBSD; version=12.2-RELEASE"

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

version/distro/distro.go

@@ -19,6 +19,9 @@ const (
 	OpenWrt  = Distro("openwrt")
 	NixOS    = Distro("nixos")
 	QNAP     = Distro("qnap")
+	Pfsense  = Distro("pfsense")
+	OPNsense = Distro("opnsense")
+	TrueNAS  = Distro("truenas")
 )
 
 // Get returns the current distro, or the empty string if unknown.
@@ -26,6 +29,9 @@ func Get() Distro {
 	if runtime.GOOS == "linux" {
 		return linuxDistro()
 	}
+	if runtime.GOOS == "freebsd" {
+		return freebsdDistro()
+	}
 	return ""
 }
 
@@ -56,3 +62,15 @@ func linuxDistro() Distro {
 	}
 	return ""
 }
+
+func freebsdDistro() Distro {
+	switch {
+	case have("/etc/pfSense-rc"):
+		return Pfsense
+	case have("/usr/local/sbin/opnsense-shell"):
+		return OPNsense
+	case have("/usr/local/bin/freenas-debug"):
+		return TrueNAS
+	}
+	return ""
+}