When launching a child backend, take care to close file descriptors for

When launching a child backend, take care to close file descriptors for
any other client connections that may exist (which would only happen if
another client is currently in the authentication cycle).  This avoids
wastage of open descriptors in a child.  It might also explain peculiar
behaviors like not closing connections when expected, since the kernel
will probably not signal EOF as long as some other backend is randomly
holding open a reference to the connection, even if the client went away
long since ...