pim: Fix autorp group joins (backport #18225)

[mergify]

Group joining got broken when moving the autorp socket to open/close as needed. This fixes it so autorp group joining is properly handled as part of opening the socket.

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This is an automatic backport of pull request #18225 done by Mergify.

[mergify]

Cherry-pick of d840560 has failed:

On branch mergify/bp/stable/10.2/pr-18225
Your branch is up to date with 'origin/stable/10.2'.

You are currently cherry-picking commit d840560b7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   pimd/pim_iface.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   pimd/pim_autorp.c
	both modified:   tests/topotests/pim_autorp/test_pim_autorp.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally