patch 8.1.0519: cannot save and restore the tag stack

Problem:    Cannot save and restore the tag stack.
Solution:   Add gettagstack() and settagstack(). (Yegappan Lakshmanan,
            closes #3604)

runtime/doc/usr_41.txt

@@ -1028,6 +1028,8 @@ Various:					*various-functions*
 
 	taglist()		get list of matching tags
 	tagfiles()		get a list of tags files
+	gettagstack()		get the tag stack
+	settagstack()		modify the tag stack
 
 	luaeval()		evaluate Lua expression
 	mzeval()		evaluate |MzScheme| expression