feature/c2n: move answerC2N code + deps out of control/controlclient

c2n was already a conditional feature, but it didn't have a
feature/c2n directory before (rather, it was using consts + DCE). This
adds it, and moves some code, which removes the httprec dependency.

Also, remove some unnecessary code from our httprec fork.

Updates #12614

Change-Id: I2fbe538e09794c517038e35a694a363312c426a2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/condregister/maybe_c2n.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_c2n
+
+package condregister
+
+import _ "tailscale.com/feature/c2n"