Remove dead state clients from state client list. Consider the following
scenario:

01 start container
02 issue shutdown request
03 state_client_fd is added to lxc_handler
03 container doesn't respond to shutdown request
04 user aborts shutdown request
05 lxc_cmd_fd_cleanup() removes state_client_fd from lxc_mainloop
06 invalid state_client_fd is still recorded in the lxc_handler
07 user issues lxc_cmd_stop() request via SIGKILL
08 container reaches STOPPED state and sends message to state_client_fd
09 state_client_fd number has been reused by lxc_cmd_stop_callback()
10 invalid data gets dumped to lxc_cmd_stop()

Reproducer:
Set an invalid shutdown signal to which the init system does not respond with a
shutdown via lxc.signal.halt e.g. "lxc.signal.halt = SIGUSR1". Then do:

1. start container
    root@conventiont|~
    > lxc-start -n a1

2. try to shutdown container
    root@conventiont|~
    > lxc-stop -n a1

3. abort shutdown
^C

4. SIGKILL the container (lxc.signal.stop = SIGKILL)
    root@conventiont|~
    > lxc-stop -n a1 -k
    lxc-stop: a1: commands.c: lxc_cmd_rsp_recv: 165 File too large - Response data for command "stop" is too long: 12641 bytes > 8192

To not let this happen we remove the state_client_fd from the lxc_handler when
we detect a cleanup event in lxc_cmd_fd_cleanup().
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>