byobu-select-session should exec on last line

[Ryan Thompson]

The last line of byobu-select-session should use exec to avoid having the stray sh process sticking around unnecessarily.

The reason I want this is because of iTerm2's new feature to allow closing a window without prompting if certain processes are running. I can whitelist byobu, tmux, screen, etc. since closing them doesn't lose my session, but iTerm still prompts because of that "sh" process, and I don't want to whitelist sh because that would allow any script to be closed without a prompt.