ipn/ipnlocal: add HTTP/2 h2c server support to peerapi on non-mobile platforms

To make ExitDNS cheaper.

Might not finish client-side support in December before 1.20, but at
least server support can start rolling out ahead of clients being
ready for it.

Tested with curl against peerapi.

Updates #1713

Change-Id: I676fed5fb1aef67e78c542a3bc93bddd04dd11fe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -46,6 +46,10 @@ import (
 
 var initListenConfig func(*net.ListenConfig, netaddr.IP, *interfaces.State, string) error
 
+// addH2C is non-nil on platforms where we want to add H2C
+// ("cleartext" HTTP/2) support to the peerAPI.
+var addH2C func(*http.Server)
+
 type peerAPIServer struct {
 	b          *LocalBackend
 	rootDir    string // empty means file receiving unavailable
@@ -492,6 +496,9 @@ func (pln *peerAPIListener) serve() {
 		httpServer := &http.Server{
 			Handler: h,
 		}
+		if addH2C != nil {
+			addH2C(httpServer)
+		}
 		go httpServer.Serve(&oneConnListener{Listener: pln.ln, conn: c})
 	}
 }