bug#36941: Please include option to link to maildir files

[Jean Louis]

I use:  Editor:      GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, X 
toolkit, Xaw3d scroll bars)

        Hyperbole:   7.0.3
        Sys Type:    x86_64-pc-linux-gnu
        OS Type:     gnu/linux
        Window Sys:  x
        News Reader: Gnus v5.13

I hope you know what are maildir messages. Each message is for itself
within a maildir folder. I know that Emacs can display such messages.

'link-to-mail'
     Displays a mail message with MAIL-MSG-ID from optional MAIL-FILE.
     See the documentation for the variable 'hmail:init-function' for
     information on how to specify the mail reader to use.

MAIL-MSG-ID is not necessary for maildir messages, as it is almost
same as link to file, the difference would be that some headers could
be hidden and message or mail mode would be turned on. Messages could
be replied, forwarded, etc.

Maybe some action can be used for maildirs, just skipping the
MAIL-MSG-ID. But it somehow need to know if it is maildir link by
asking the user.

Jean