remctl_close() cleanly closes any connection to a remote server created
via remctl_open() for the given client object and then frees the object
created by remctl_new(). It should be called when the caller is finished
with a remctl client object to avoid resource leaks. Following the call
to remctl_close(), the r pointer to the remctl client object is no
longer valid.
remctl_close() is always successful, even if it is unable to send a clean
protocol quit command to the remote server.