[RFC] add experimental support for unionfs V2

Logic of unionfs
 I save logic from default link mod, because he beautiful work. I just mount in /tmp/$user-$browser unionfs, instead of do rsync. Changes on unionfs write in /tmp/$user-$browser-rw, source of files read from /home/$user/.config/$browser.
(this is example of logic, you can install unionfs-fuse and test this, i test my new solution about 2 days and he work nice, because it clean and simple and also based on default logic of psd).

If unionfs not installed and option enabled, psd still work in link case, so also allow add unionfs-fuse to optional depends.
Adding of unionfs get speedup start time.
Tests (vertex 4 128G, btrfs) in profile I create big file by truncate -s 1G ~/.config/chromium/lol
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.

Also on my big pc, with hdd, unionfs speed up boot time, from about 52 seconds to 32 seconds.
P.S. I add this TMP="$VOLATILE/$user-$browser$suffix", to fit code in 80 symbols.