monitor: Clean up fd sets on monitor disconnect

Fd sets are shared by all monitor connections.  Fd sets are considered
to be in use while at least one monitor is connected.  When the last
monitor disconnects, all fds that are members of an fd set with no
outstanding dup references are closed.  This prevents any fd leakage
associated with a client disconnect prior to using a passed fd.

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
