Keeps the attachment file names stable while it also allows to drop the dep on Tempfile, the unveil of /tmp and the fattr pledge promise too.

The idea is to use this cache to spot changed threads and re-export all the mail of the threads that changed. This will allow to add thread navigation hints on each exported mail and have them up-to-date. This makes the "skip if already exported" logic in mexp redundant and incomplete. pe will decide whether to export or not, and mexp blindly do the work.

the children are pre-forked, so at runtime only "stdio" is needed.

avoids the "Wide character in print at ./mkindex line 152, <> line 2751" warnings.

pick up the *latest* From/Date for the indexes, not what happened to be the last mail rendered in that thread. spotted by stsp@

Instead, parse the output of `mshow -t' and extract, one by one, the interesting parts. This allows to generate a better listing (we know name and size!) and to decide how to call the files (the extension is important for httpd!)

reminded by semarie@, thanks!

we can't use the tmppath pledge promise as File::Temp checks whether /tmp exists. We could unveil("/tmp", "r") and use `tmppath', but it seem misleading. (noticed after semarie@ reminded me of `tmppath')