Don't start syncthing manually

Use systemd service units where applicable, elsewhere use stman (in
host-specific branch).

dotfiles/i3/config

 exec "shutter --min_at_startup"
 exec "tasknag -R"
 exec "tasknag -n"
-exec "syncthing -no-browser -logfile .local/share/syncthing/syncthing.log >/dev/null"
 exec "QSyncthingTray"