These programs don't need to read files in the current directory: GotMArc loads in memory some files at load time, at that point no other accesses to "." are needed.

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.