Warning if used without --new-session

With version 0.1.7 the unconditional call of setsid() has been replaced with the --new-session option and the hint to alternativly use seccomp filter to neutralize the TIOCSTI issue.

However i feel like this is somewhat risky, as few users will read the release notes and consequently use the new option. Using something like --no-new-session and calling setsid by default would be a more secure approach, but I suspect this is not wanted.

One idea to solve this would be to display a warning if bwrap is used without --new-session.