Use original arguments on arbiter re-exec, and tell systemd so exiting old master does not terminate service

[pajod]

• missing: document difference between behavior of < 3.10 and Python >= 3.10

• missing: document Type=notify-reload (systemd >= v253)

• missing: #2856 -- automatically shutdown old master after handling USR2 re-exec while systemd integration is active (because that is what you expect when you run systemctl reload gunicorn)

• Fixes: #3281

• Fixes: #1948