I've been having problems adding a maildir source with a space in its absolute path.

  sup-add "maildir:/path/to/maildir with spaces"

It works if you enscape spaces with %20 or + as done normally in URIs:

  sup-add "maildir:/path/to/maildir%20with%20spaces"

But the maildir source doesn't unescape the uri path. Attached patch fixes that. 

However, I'm still not convinced one should be expected to escape URIs for
sup-add. I'd be interested in your opinion on that matter, and willing to go and
write a patch for that part as well.