start instance; kill mysqlmanager; show ...

The problem was that Instance Manager didn't close client
sockets (sockets for client connections) on execing mysqld
instance. So, mysqld-instance inherits these descriptors.

The fix is to set close-on-exec flag for each client socket.


mysql-test/r/im_daemon_life_cycle.result:
  Updated result file.
mysql-test/t/im_daemon_life_cycle.imtest:
  Test for BUG#12751.
server-tools/instance-manager/listener.cc:
  Set close-on-exec flag for each client socket.