runc-docker: Allow "run start ..." to daemonize with $SIGUSR1_PARENT_PID

The runc-docker has all the code in it to properly run a stop hook if
you use it in the foreground.  It doesn't work in the back ground
because there is no way for a golang application to fork a child exit
out of the parent process because all the golang threads stay with the
parent.

This patch has three parts that happen ONLY when $SIGUSR1_PARENT_PID
is set.

1) At the point where runc start would normally exit, it closes
   stdin/stdout/stderr so it would be possible to daemonize "runc start ...".

2) The code to send a SIGUSR1 to the parent process was added.  The
   idea being that a parent process would simply exit at that point
   because it was blocking until runc performed everything it was
   required to perform.

3) The code was copied which performs the normal the signal handling
   block which is used for the foreground operation of runc.

-- More information --

When you use "runc run " it is running in the "foreground", in the
sense it takes over your existing terminal.

The runc-docker doesn't have a way to start it with "runc run&" where
you can send it to the background and have everything work.  With this
commit, it does allow you to do that and have all the stop hooks fire
at the time what ever runc started exits.

Lets take a quick look at what "runc run" does today:

  * Starts a whole pile of threads
  * Sets up all name spaces
  * Starts child process for container and leaves it paused at image activation
  * runs start hooks
  * executes "continue" for container process
  * waits for container app to exit
  * executes stop hooks

Now lets look at "runc create/start" does today:
   runc create
     * Starts a whole pile of threads
     * Sets up all name spaces
     * Starts child process for container and leaves it paused at image activation
     * exits  -- [ NOTE: this is our problem! ]
   runc start
     * runs start hooks
     * executes "continue" for continue process

   At this point when the container app exits nothing is waiting for it
   to run any kind of hooks.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>

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