ui/ncurses: Always cancel autoboot on exit

If the ncurses UI exits before it has contacted the server it is meant
to fork a process to wait until the connection is made and cancel
autoboot. This prevents users dropping to the shell and then having the
machine boot out from underneath them.

In e1e2ca68 "Spawn shell in exit handler" the UI doesn't actually exit
on "exit", but it isn't listening to server events either while the
shell is active. In this case make sure we still fork to notify the
server.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>