iostream: shutdown socket fd before close

If a socket fd is shared by a child process, closing it in the parent
will not shut down the underlying connection. As a result, the server
may hang executing the graceful shutdown protocol. Fix this problem by
explicitly shutting down the connection socket fd before closing it.

This is a recommended way to terminate a Unix socket connection, see
http://www.faqs.org/faqs/unix-faq/socket/#:~:text=2.6.%20%20When%20should%20I%20use%20shutdown()%3F

Closes #7256

NO_DOC=bug fix