DEBUG: cli: support closing "hard" using close() in addition to fd_delete()

DEBUG: cli: support closing "hard" using close() in addition to fd_delete()

"debug dev close <fd>" currently closes that FD using fd_delete() after
checking that it's known from the fdtab. Sometimes we also want to just
perform a pure close() of FDs not in the fdtab (pollers, etc) in order
to provoke certain error cases. The optional "hard" argument to the
command will make it use a plain close() instead of fd_delete() and skip
the fd owner check. The main visible effect when closing a traffic socket
with it is that instead of dying from a double fd_delete() by seeing that
fd.owner is already 0, it will die during the next fd_insert() seeing that
fd.owner was not 0.