bug#35249: Allow easier customization of message-unique-id

[Robert Pluim]

>>>>> On Sat, 13 Apr 2019 00:07:54 +0800, 積丹尼 Dan Jacobson 
>>>>> <jidanni@jidanni.org> said:

    Dan> As concat ".fsf" is hardwired into message-unique-id, I had
    Dan> to redefine the entire function, just to make that instead
    Dan> into "." (substring (system-name) -1 nil) ".fsf" So do
    Dan> consider allowing the user to customize this easier.

So would a simple string suffice? Or would you want to be able to run
arbitrary lisp code to generate it?

Robert