Add ActivatingConcurrencyMax for slice startup pacing

Add ActivatingConcurrencyMax for slice startup pacing

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.