nohup: use disown instead

Now the backgrounded process isn't even *in* the process tree of the shell,
meaning `wait(1)` and friends won't block on it.
Merge disown-instead-of-nohup (#6) into master

nohup: use disown instead

Now the backgrounded process isn't even *in* the process tree of the
shell, meaning `wait(1)` and friends won't block on it.