Add experimental unionfs support

Add support for unionfs
  Backup folder mounting as RO.
  TMP folder mounted for RW.
  Profile folder mounted as unionfs.
Unionfs code separeted in another block.
If unionfs not installed and option enabled, psd still work in default case so allow add unionfs-fuse to optional depends.
Adding of unionfs get speedup start time.
Test
Tests
size of 1.1G /home/timofey/.config/chromium
command          Unionfs  Enabled Disabled
sudo systemctl start psd  1.567   13.067  sec
sudo systemctl stop  psd  1.610   1.588   sec

By default experimental features disabled.