When the main arbiter crashes and restarts, orphaned dirty arbiters
may continue running. This adds detection and cleanup:
- Add well-known PID file location based on proc_name
- Dirty arbiter writes PID on startup, removes on exit
- Main arbiter checks for orphans on fresh start (not USR2)
- Uses self.proc_name for USR2 compatibility (myapp vs myapp.2)
During USR2 upgrade, old and new dirty arbiters coexist with
separate PID files, preventing the old from removing the new's file.
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