Introduce Slice.ActivatingConcurrencyMax to limit how many units
within a slice hierarchy may be in activating state concurrently.
Expose the setting over D-Bus, support transient/property parsing,
enforce it during unit start dispatch, and re-check queued starts when
units leave activating state.
Document the new slice option and add a PID1 concurrency test covering
queued startup behavior.
Morteza Pourkazemi
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