Instead of excluding one checkbox at a time, only check what should be checked

Instead of excluding one checkbox at a time, only check what should be checked
by Toggle All.

This will allow plugins to probably put some more checkboxes (like plugin
move_all_messages).

Of course, the Toggle All link needs to be restored at some point too. (When did
that go away in -DEVEL?)

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@7297 7612ce4b-ef26-0410-bec9-ea0150e637f0