feature/featuretags: add features for c2n, peerapi, advertise/use routes/exit nodes

Saves 262 KB so far. I'm sure I missed some places, but shotizam says
these were the low hanging fruit.

Updates #12614

Change-Id: Ia31c01b454f627e6d0470229aae4e19d615e45e3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/forwarder.go

@@ -27,6 +27,7 @@ import (
 	dns "golang.org/x/net/dns/dnsmessage"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/envknob"
+	"tailscale.com/feature"
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health"
 	"tailscale.com/net/dns/publicdns"
@@ -530,6 +531,9 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 		}()
 	}
 	if strings.HasPrefix(rr.name.Addr, "http://") {
+		if !buildfeatures.HasPeerAPIClient {
+			return nil, feature.ErrUnavailable
+		}
 		return f.sendDoH(ctx, rr.name.Addr, f.dialer.PeerAPIHTTPClient(), fq.packet)
 	}
 	if strings.HasPrefix(rr.name.Addr, "https://") {