Race condition when using ControlMaster=auto with simultaneous connections (added Bug 3971 and a patch; requesting review)

[Jens Rosenboom]

Hello OpenSSH developers,
(hello Baptiste Jonglez)

I opened*Bug 3971**(* <https://bugzilla.mindrot.org/show_bug.cgi?id=3971>Race condition when using ControlMaster=auto with simultaneous 
connections) <https://bugzilla.mindrot.org/show_bug.cgi?id=3971>: There is a (kind of harmless) race condition in the connection multiplexing code when multiple connections to a new host happen at the same time.
The connections end up not using connection multiplexing at all.
Baptiste Jonglez initially reported this issue (back in 2022, on Wed Aug 31 23:24:12 AEST, see ongroups.google.com), but there wasn't much response to it.

It would be great if anyone could give feedback on the patch, or maybe even apply it upstream.
Thank you & Best regards,
Jens Rosenboom