Remove check for exited child processes

Since daemonize is the default, remove the check for child processes.
However, it seems daemonize mode starts up lxc-monitord which has its
own problems.
Notifications for state changes

liblxc uses an abstract socket to poll for state changes. Add a
simple mechanism for notifying the erlang process of state changes.
Doesn't appear to be a way of subscribing to events, e.g., selecting on
the command socket for the container.

For now, a Unix process polls the sockets at an interval specified by
the user (in seconds). The poll process will also check every time a
message is sent to the port.
Remove check for NULL container reference

Since the port now allocates the container structure on startup, no
point in checking if the container is NULL. The code will crash when the
container functions are run at exit.

The old code multiplexed containers from a single port so containers
could be NULL. Note some liblxc functions (lxc_container_put) have the
side effect of freeing the reference but do not set it to NULL.
Log reason for malloc() failures