mountlib: correctly daemonize for compatibility with automount - #5593

This patch will:
- add --daemon-wait flag to control the time to wait for background mount
- remove dependency on sevlyar/go-daemon and implement backgrounding directly
- avoid setsid during backgrounding as it can result in race under Automount
- provide a fallback PATH to correctly run `fusermount` under systemd as it
  runs mount units without standard environment variables
- correctly handle ^C pressed while background process is being setting up

fs/daemon_other.go

@@ -0,0 +1,11 @@
+// Daemonization stub for non-Unix platforms (common definitions)
+
+//go:build windows || plan9 || js
+// +build windows plan9 js
+
+package fs
+
+// IsDaemon returns true if this process runs in background
+func IsDaemon() bool {
+	return false
+}