logpolicy: automatically figure out paths and filenames.

The autoselection should pick sensible paths for all of:
 - Windows (LocalAppData)
 - Mac (Library/Caches)
 - Unix user (XDG_CACHE_DIR)
 - Linux systemd service (CACHE_DIRECTORY)

As a last resort, if cache dir lookup fails, plops sufficiently
uniquely named files into the current working directory.

Signed-off-by: David Anderson <dave@natulte.net>

cmd/tailscaled/tailscaled.go

@@ -46,7 +46,7 @@ func main() {
 	if err != nil {
 		logf("fixConsoleOutput: %v\n", err)
 	}
-	pol := logpolicy.New("tailnode.log.tailscale.io", "tailscaled")
+	pol := logpolicy.New("tailnode.log.tailscale.io")
 
 	getopt.Parse()
 	if len(getopt.Args()) > 0 {