auto-boot on systemd

[aleprovencio]

Hello, just for you information @continuum-boot wouldn't create the correct service on my arch system until a manual mkdir -p systemd/user command.

[andy-twosticks]

Just tried to turn this on, and got exactly the same (non-)behaviour: no service was created.

Tried again without @continuum-restore (because of #36) but no change.

Debian Jessie, Tmux 2.2.