Simplify folder_exists() to just testing stat(2)'s return value.

[nmh] / docs / historical / mh-6.8.5 / support / general / scan.size

%4(msg)%<(cur)+%| %>%<{replied}-%?{encrypted}E%| %>\
%02(mon{date})/%02(mday{date})%<{date} %|*%>\
%5(size) \
%<(mymbox{from})%<{to}To:%14(friendly{to})%>%>%<(zero)%17(friendly{from})%>  \
%{subject}%<{body}<<%{body}%>