Use a dash-truncated drop-in for user-%j.slice configuration

Use a dash-truncated drop-in for user-%j.slice configuration

This removes the UserTasksMax= setting in logind.conf. Instead, the generic
TasksMax= setting on the slice should be used. Instead of a transient unit we
use a drop-in to tweak the default definition of a .slice. It's better to use
the normal unit mechanisms instead of creating units on the fly. This will also
make it easier to start user@.service independently of logind, or set
additional settings like MemoryMax= for user slices.

The setting in logind is removed, because otherwise we would have two sources
of "truth": the slice on disk and the logind config. Instead of trying to
coordinate those two sources of configuration (and maintainer overrides to
both), let's just convert to the new one fully.

Right now now automatic transition mechanism is provided. logind will emit a
hint when it encounters the setting, but otherwise it will be ignored.

Fixes #2556.