two-step: set is_idle=TRUE at shutdown time

two-step: set is_idle=TRUE at shutdown time

Right now, we do the end animation immediately if in
shutdown mode.  This is because shutdown is fast,
and we don't try to estimate how long it will take.

The code depends on is_idle being true after the end
animation is run, but we neglect to do that in the
shutdown case.

This commit makes shutdown mode just call
"become_idle" right away, and deletes the duplicated
shutdown code path.