buffering: Don't execute move-handle-ony case if handle is of metadata type (atomic...

buffering: Don't execute move-handle-ony case if handle is of metadata type (atomic) and must be kept fully buffered. Manage handle corruption guard and handle buffering with one set of logic which allows reading of the maximum amount of data without overflow. 'FIXME' regarding handle corruption guard is really part of expected operation when thread that does the handle closing hasn't yet performed the delegated task before rebuffering starts.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29270 a1c6a512-1295-4272-9138-f99709370657