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.

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!

in theory `mexp' could use the tmppath pledge promise and drop unveil("/tmp/", "rwc") BUT File::Temp checks whether /tmp exists. Another option would be to unveil("/tmp", "r") and use tmppath, but that could seem misleading. (noticed after semarie@ reminded me of `tmppath')

while here also drops a useless chomp

there's little point to use pledge/unveil and then allow the program to modify itself...