logind: add a new UserTasksMax= setting to logind.conf

logind: add a new UserTasksMax= setting to logind.conf

This new setting configures the TasksMax= field for the slice objects we
create for each user.

This alters logind to create the slice unit as transient unit explicitly
instead of relying on implicit generation of slice units by simply
starting them. This also enables us to set a friendly description for
slice units that way.