patch 8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes #3652)

src/workshop.c

@@ -491,7 +491,7 @@ workshop_get_mark_lineno(
     lineno = 0;
     buf = buflist_findname((char_u *)filename);
     if (buf != NULL)
-	lineno = buf_findsign(buf, markId);
+	lineno = buf_findsign(buf, markId, NULL);
 
     return lineno;
 }