Allow to create tarpc server on existing listener

[SabatierBoris]

When you want to use systemd.socket or systemfd for allocate the socket (TCP or UNIX) you need to be able to create a server on already existing listener

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[tikue]

Thanks, and apologies for the review delay! Can you add unit tests for the new functions? Other than that, looks great.

[SabatierBoris]

Thanks, and apologies for the review delay! Can you add unit tests for the new functions? Other than that, looks great.

Hi, no problem.

I add some tests based on existing TCP and UDS tests.