Retain argv + fds on re-exec, fix USR2 under systemd by notifying new PID

[pajod]

This PR stops mangling sys.argv and socket fd numbers on re-exec, thus permitting much improved integration with systemd.

• While not meant to break anything before that, this patch is only effective in cPython >= 3.10.
  • Python docs explicitly say that relaunching with -m switch is the recommended procedure, so that should work

diff --git a/gunicorn/arbiter.py b/gunicorn/arbiter.py
-        args = sys.argv[:]
-        args.insert(0, sys.executable)
+        if sys.version_info < (3, 10):
+            args = sys.argv[:]
+            args.insert(0, sys.executable)
+        else:
+            args = sys.orig_argv[:]

• New feature: send timestamp to properly synchronize reload cycles using Type=notify-reload (systemd >= v253)

• Fixes: module invocation breaks USR2 upgrading, as modified argv results in modified path #3281

• Fixes: more status to systemd #1948

• Fixes: Compatibility of systemd with zero downtime reloads with SIGUSR2 #2856

  • insofar as systemctl -s USR2 gunicorn.service now automatically issues graceful quit
  • systemctl reload gunicorn.service and systemctl restart gunicorn.service unchanged in .service example

• Improves systemd-specific case of: Error instead of warning in logs when sending SIGTERM since v21 #3050

• Improves sock fd leak pointed out in: fd leak in thread worker causing periodic outages #3339

• Follow-Up plans not included here:

  • document difference between behavior of < 3.10 and Python >= 3.10
  • could be improved with BARRIER=1 so reloading/re-execing in quick success is reliable as well
  • could be improved with EXTEND_TIMEOUT_USEC= so automatic timeouts are reliable as well