Reverse earlier "allow tag without 'tag:' prefix" changes.

These accidentally make the tag syntax more flexible than was intended,
which will create forward compatibility problems later. Let's go back
to the old stricter parser.

Revert "cmd/tailscale/cli: fix double tag: prefix in tailscale up"
Revert "cmd/tailscale/cli, tailcfg: allow tag without "tag:" prefix in 'tailscale up'"

This reverts commit a702921620f7b6e386f393a9a1340d4218597469.
This reverts commit cd07437ade.

Affects #861.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

cmd/tailscale/cli/up.go

@@ -182,19 +182,11 @@ func runUp(ctx context.Context, args []string) error {
 	var tags []string
 	if upArgs.advertiseTags != "" {
 		tags = strings.Split(upArgs.advertiseTags, ",")
-		for i, tag := range tags {
-			if strings.HasPrefix(tag, "tag:") {
-				// Accept fully-qualified tags (starting with
-				// "tag:"), as we do in the ACL file.
-				if err := tailcfg.CheckTag(tag); err != nil {
-					fatalf("tag: %q: %v", tag, err)
-				}
-				continue
+		for _, tag := range tags {
+			err := tailcfg.CheckTag(tag)
+			if err != nil {
+				fatalf("tag: %q: %s", tag, err)
 			}
-			if err := tailcfg.CheckTagSuffix(tag); err != nil {
-				fatalf("tag: %q: %v", tag, err)
-			}
-			tags[i] = "tag:" + tag
 		}
 	}