Add Maildir support to RMAIL

[csh]

Fellow Emacsites,

I really love the simplicity of reading mail with RMAIL, but most email 
providers no longer use the mbox format for mail.  Is there any way someone 
could get RMAIL working with Maildir mailboxes?

The only thing I could find on the matter was a blog entry[1], but the solution 
did not seem complete.  It was more of a workaround, and I don't know how well 
it would work day-to-day.

Thanks,
Caleb Herbert

CC: Antoine Kalmbach.

[1]: Kalmbach, Antoine.  "Back to Rmail."  URI: 
<https://ane.github.io/2020/09/09/rmail.html>.