mandos: Avoid closing configured server network socket
If we are passed a pre-created created network socket as a file descriptor, we must avoid later closing it (when stdin, stdout and stderr are dup2()ed over with /dev/null) by making sure that the network file descriptor is not 0, 1, or 2, by creating a new file descriptor if necessary.
However, when we create a new file descriptor we must check that the new file descriptor is not 0, 1, or 2 either, and try again until we get a suitable file descriptor.