runcon: revert "disable use of the TIOCSTI ioctl"

runcon: revert "disable use of the TIOCSTI ioctl"

This reverts commit v8.27-97-g8cb06d4 because
the setsid() fallback was not implemented correctly
and disabling the ioctl was not a complete solution
to the security issue of the child being passed
the tty of the parent.

Given runcon is not really a sandbox command,
the advice is to use `runcon ... setsid ...`
to avoid this particular issue.