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>

feature/buildfeatures/feature_useexitnode_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_useexitnode
+
+package buildfeatures
+
+// HasUseExitNode is whether the binary was built with support for modular feature "Use exit nodes".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_useexitnode" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasUseExitNode = true