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>