lxc-execute to return exit code of its child not others

lxc-execute to return exit code of its child not others

lxc-execute has to return the exit code of it's child not others
as today's code would return the exit code of the last child.
We need to track the first process we launched and store its
exit status when it exits. In order to avoid to detect the exit
of this pid several time if the pids number wrap, we put an extra
flag saying the process group is orhpan.

Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>