maildir: Keep trailing comma even when no flags are set

Without this, `mk_info_filename' generates filenames of the following
form when the flag set is empty:

    1402217787.M664911P5957Q143.hostname:2

The original Maildir spec (http://cr.yp.to/proto/maildir.html),
however, mandates that filenames must be of the form `uniq:info', with
`info' starting with "2,"--including a comma.

This patch removes the "cleanup" performed by `mk_info_filename' as
that there is no rationale for it in mailutils' source code history,
and as it breaks Gnus' "nnmaildir" backend (Cf.  `nnmaildir--add-flag').

* libproto/maildir/mbox.c (mk_info_filename): Keep comma at the end of
filename when the flags suffix is empty.

@@ -278,10 +278,7 @@ mk_info_filename (char *directory, char *suffix, char *name, int flags)
   size += 3 + strlen (fbuf);
 
   tmp = malloc (size);
-  if (!fbuf[0])
-    sprintf (tmp, "%s/%s/%*.*s:2", directory, suffix, namelen, namelen, name);
-  else
-    sprintf (tmp, "%s/%s/%*.*s:2,%s", directory, suffix, namelen, namelen, name, fbuf);
+  sprintf (tmp, "%s/%s/%*.*s:2,%s", directory, suffix, namelen, namelen, name, fbuf);
   return tmp;
 }