version: unexport all vars, turn Short/Long into funcs

The other formerly exported values aren't used outside the package,
so just unexport them.

Signed-off-by: David Anderson <danderson@tailscale.com>

ipn/ipnlocal/cert.go

@@ -261,7 +261,7 @@ func (b *LocalBackend) getCertPEM(ctx context.Context, logf logger.Logf, traceAC
 	}
 	ac := &acme.Client{
 		Key:       key,
-		UserAgent: "tailscaled/" + version.Long,
+		UserAgent: "tailscaled/" + version.Long(),
 	}
 
 	a, err := ac.GetReg(ctx, "" /* pre-RFC param */)