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>

envknob/envknob.go

@@ -485,5 +485,5 @@ func IPCVersion() string {
 	if v := String("TS_DEBUG_FAKE_IPC_VERSION"); v != "" {
 		return v
 	}
-	return version.Long
+	return version.Long()
 }