execute: ensure parent is notified about child exec and close all unneeded fds

execute: ensure parent is notified about child exec and close all unneeded fds

lxc_container_init() creates the container payload process as it's child
so lxc_container_init() itself never really exits and thus the parent
isn't notified about the child exec'ing since the sync file descriptor
is never closed. Make sure it's closed to notify the parent about the
child's exec.

In addition we're currently leaking all file descriptors associated with
the handler into the stub init. Make sure that all file descriptors
other than stderr are closed.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>