Use a real parser to extract headers instead of regexes

[n-peugnet]

For instance formail(1), or, more likely reformail(1), or maybe even a custom one, maybe based on eml-codec.

Why reformail more than formail, see this article : https://anarc.at/blog/2022-03-02-procmail-considered-harmful/

formail(1) is similar: the courier maildrop package ships reformail(1) which is, presumably, a rewrite of formail. It's unclear if it's a drop-in replacement, but it should probably possible to port uses of formail to it easily.

But there is still an inconvenience:

Update: the maildrop package ships a SUID root binary (two, even). [...] It would be perhaps better to have reformail(1) as a separate package, see bug 1006903 for that discussion.

Hence the idea to create a simple CLI program based on eml-codec.