fix(jailer): Make "init" in new PID NS a session leader

https://man7.org/linux/man-pages/man7/pid_namespaces.7.html
> A process in an ancestor namespace can send signals to the "init"
> process of a child PID namespace only if the "init" process has
> established a handelr for that signal.

Firecracker (i.e. the "init" process of the new PID namespace) sets up
handlers for some signals including SIGHUP and jailer exits soon after
spawning firecracker into the new PID namespace. If the jailer process
is a session leader and its exit happens after firecracker configures
the signal handlers, SIGHUP will be sent to firecracker and be caught by
the handler unexpectedly.

In order to avoid the above issue, if jailer is a session leader,
creates a new session and makes the child process (i.e. firecracker)
become the leader of the new session to not get SIGHUP on the exit of
jailer.

Note that this is the case only if `--daemonize` is not passed. This is
because we use the double fork method to daemonize, making itself not a
session leader.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86