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

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

%<(cur)>%| %>\
%<{status} %|N%>\
%<{replied}R%?{encrypted}E%| %>\
%4(msg)\
 %<(mymbox{from})%<{to}To: %13(friendly{to})%>%>%<(zero)%17(friendly{from})%>\
 %3(day{date}) %3(month{date}) %02(mday{date})\
 %02(hour{date}):%02(min{date})\
 %{subject}