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)
diff --git a/src/workshop.c b/src/workshop.c
index 7b09e33..3de4df1 100644
--- a/src/workshop.c
+++ b/src/workshop.c
@@ -491,7 +491,7 @@
     lineno = 0;
     buf = buflist_findname((char_u *)filename);
     if (buf != NULL)
-	lineno = buf_findsign(buf, markId);
+	lineno = buf_findsign(buf, markId, NULL);
 
     return lineno;
 }