* Add new function `smex-update-if-needed` and use it where
* Rename `update-smex-after` to `smex-auto-update-after`
* Advice defined by `smex-auto-update-after` now only works when
`smex-auto-update' is set to t.
* Other minor cleanups
These are all in one commit because they all affect overlapping lines,
so separating them out would have been quite difficult.