Do not shutdown() an external server socket

[fedegiova]

For the socket-activation use case the external socket should not be shutdown() when the TServerSocket is closed.

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[fedegiova]

Issue found during testing with a systemd socket activated service. Without the patch the journal will report:

Jan 26 12:30:10 pegasus-base-6434 systemd[1]: display-service.socket: Got POLLHUP on a listening socket. The service probably invoked shutdown() on it, and should better not do that.