tools: unify exit() calls outside of main()

tools: unify exit() calls outside of main()

This also changes the help exit status of lxc-unshare and lxc-usernsexec. So
far they did exit(EXIT_FAILURE) whereas all other tools do exit(EXIT_SUCCESS).
Let's align them with the rest of the tools. This should be safe to do.

Signed-off-by: Christian Brauner <cbrauner@suse.de>