Sets up a login screen using tuigreet. From there the user is thrown
into a shell. Other units/services (usually, the user service `river`)
take care to start an actual wayland session for the user.
User services under control of runit under turnstiled now default to the
`~/.local/state/service/` directory (and `~/.local/state/service-env/`
for the env vars). This mimics the system-wide runit setup, with service
templates kept in `/etc/sv` (in my case `~/.config/sv` for user
services) and active ones in `/var/service`