Fix sidebar references to the "new count" to be "unread". (closes #3908)

Fix sidebar references to the "new count" to be "unread". (closes #3908)

%N in $sidebar_format and "new"in $sidebar_sort_method actually use
the unread message count.  Update the documentation to mention that.

Add an "unread" method to $sidebar_sort_method, but preserve "new" for
compatibility.

Change the SORT_COUNT_NEW constant to SORT_UNREAD, so the code is also
consistent with the meaning.

Thanks to cri for reporting the problem and suggesting where to fix it
in the code.