Use shutdown() ahead of close() for rlogin/ssh

Using only close() on a socket has a few issues:

 (1) poll is not required to wake on such events leading to hangs
     if infinite timeouts are used.

     This can be observed in macOS:

     (a) syncterm -iC
     (b) connect via telnet to some system
     (c) use the menu to disconnect from the system

     Note: socket must be idle to reproduce.

 (2) If a blocked recv/send receives an error in response to a close
     from another thread, the context of the socket has been lost: the
     descriptor is free to be reallocated under the thread performing
     I/O by the system. Any retry logic that may be written in response
     to the error condition could lead to operations being performed
     on a valid but incorrect resource.

 (3) Processes that inherited the socket via fork() will still have
     a valid descriptor to the socket. i.e., the socket does not
     globally shutdown via a call to close(); however, shutdown()
     acts on the connection by triggering a TCP FIN. This will lead
     to the actual shutdown of the socket by the remote connection
     as well.

Solution:

Use shutdown() ahead of close(). This will initiate a proper shutdown
sequence on the socket while leaving the descriptor open. poll() will
wake, and a subsequent read or write will yield the desired EOF/EPIPE.
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