client/tailscale, cmd/tailscale/cli: move version mismatch check to CLI

So people can use the package for whois checks etc without version
skew errors.

The earlier change faa891c1f2 for #1905
was a bit too aggressive.

Fixes #2757

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -19,6 +19,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 	"text/tabwriter"
 
@@ -82,6 +83,13 @@ func Run(args []string) error {
 		args = []string{"version"}
 	}
 
+	var warnOnce sync.Once
+	tailscale.SetVersionMismatchHandler(func(clientVer, serverVer string) {
+		warnOnce.Do(func() {
+			fmt.Fprintf(os.Stderr, "Warning: client version %q != tailscaled server version %q\n", clientVer, serverVer)
+		})
+	})
+
 	rootfs := flag.NewFlagSet("tailscale", flag.ExitOnError)
 	rootfs.StringVar(&rootArgs.socket, "socket", paths.DefaultTailscaledSocket(), "path to tailscaled's unix socket")