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_unix.go

@@ -0,0 +1,21 @@
+// Daemonization interface for Unix platforms (common definitions)
+
+//go:build !windows && !plan9 && !js
+// +build !windows,!plan9,!js
+
+package fs
+
+import (
+	"os"
+)
+
+// We use a special environment variable to let the child process know its role.
+const (
+	DaemonMarkVar   = "_RCLONE_DAEMON_"
+	DaemonMarkChild = "_rclone_daemon_"
+)
+
+// IsDaemon returns true if this process runs in background
+func IsDaemon() bool {
+	return os.Getenv(DaemonMarkVar) == DaemonMarkChild
+}