control/controlclient, version/distro: detect NixOS explicitly

The fallthrough happened to work in controlclient already due to the
/etc/os-release PRETTY_NAME default, but make it explicit so it
doesn't look like an accident.

Also add it to version/distro, even though nothing needs it yet.

control/controlclient/netmap_test.go

@@ -6,6 +6,7 @@ package controlclient
 
 import (
 	"encoding/hex"
+	"encoding/json"
 	"testing"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
@@ -282,3 +283,15 @@ func TestConciseDiffFrom(t *testing.T) {
 		})
 	}
 }
+
+func TestNewHostinfo(t *testing.T) {
+	hi := NewHostinfo()
+	if hi == nil {
+		t.Fatal("no Hostinfo")
+	}
+	j, err := json.MarshalIndent(hi, "  ", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %s", j)
+}