don't tag subfolders of Drafts,Trash and Sent as special in folder management

don't tag subfolders of Drafts,Trash and Sent as special in folder management
page. Allows rename and delete operations with subfolders.

don't treat INBOX.Trash as special on Courier. Trash folder can have subfolders
in courier. Only XMAGICTRASH extension can cause errors when trash folder is
deleted. Courier does not document commands that allow to detect folder that
is used as magic trash.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11466 7612ce4b-ef26-0410-bec9-ea0150e637f0