Clean-up theme settings submit

After some struggle with my custom theme and doing a bit of research on custom theme settings (which led to posting issue http://drupal.org/node/199693 later recognized as duplicate by myself - don't even bother to click), I spotted another problem as a side effect:

When the theme settings form is submitted (i.e. admin/build/themes/settings/theme_name), the resulting theme-settings stored inside variable 'theme_theme_name_settings' contain some FAPI crap that should not be there ('op', 'form_build_id', and 'form_token'). Some elements are already removed, but there are apparently new ones in FAPI after the code has been last updated.

Additionally, the submit handler keeps some constructs inherited from 5.x, which I don't see as good practices in current 6.x code:
- relies on $_POST directly, although relevant data are now passed-in from FAPI
- performs unset() on $form_state, which is now passed by reference, just to exclude elements from data processed later in the local code. My suspicion is, that this is even done without examining possible (future?) consequences; the code seems to be completely unchanged from 5.x.

How to reproduce: Submit the theme settings form at admin/build/themes/settings/theme_name. Observe the junk in variable 'theme_[theme_name]_settings'. Also see the code of system_theme_settings_submit() inside system.admin.inc (or in the patch here).

The patch cleans this up, by using 'op' passed from FAPI rather than $_POST, doing the unsets on a local copy of the values, and of course doing it for _all_ the unnecessary elements now present. (Keeping status 'normal' rather than 'minor', since we really shouldn't insert junk into variables - unserialized on every page an so on; the rest is just cleanup).