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/taillogin/taillogin.go

@@ -31,7 +31,7 @@ func main() {
 	if *config == "" {
 		log.Fatal("no --config file specified")
 	}
-	pol := logpolicy.New("tailnode.log.tailscale.io", *config)
+	pol := logpolicy.New("tailnode.log.tailscale.io")
 	defer pol.Close()
 
 	cfg, err := loadConfig(*config)