Code for creating custom sidebars and then selecting one on a given post...

Also cleaned up some minor E_NOTICE-level PHP errors on the normal sidebar handling.

The execution of this on the Theme Options page isn't super-elegant — it's just a textarea with each custom sidebar needing to go on a separate line — but the Theme Options Framework doesn't really have any better field type to use for this.

Works fine, though. Post authors only get to choose from the custom-defined sidebars from the Theme Options page, which should mean only the sorts of folks you'd want to have the power to create new sidebars can do so.