When starting the gateway with --replace, concurrent invocations could
leave multiple instances running simultaneously. This happened because
write_pid_file() used a plain overwrite, so the second racer would
silently replace the first process's PID record.
Changes:
- gateway/status.py: write_pid_file() now uses atomic O_CREAT|O_EXCL
creation. If the file already exists, it raises FileExistsError,
allowing exactly one process to win the race.
- gateway/run.py: before writing the PID file, re-check get_running_pid()
and catch FileExistsError from write_pid_file(). In both cases, stop
the runner and return False so the process exits cleanly.
Fixes#11718
cbe29db774