feature, net/tshttpproxy: pull out support for using proxies as a feature

Saves 139 KB.

Also Synology support, which I saw had its own large-ish proxy parsing
support on Linux, but support for proxies without Synology proxy
support is reasonable, so I pulled that out as its own thing.

Updates #12614

Change-Id: I22de285a3def7be77fdcf23e2bec7c83c9655593
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -42,7 +42,6 @@ import (
 	"tailscale.com/net/netx"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tsdial"
-	"tailscale.com/net/tshttpproxy"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tempfork/httprec"
 	"tailscale.com/tka"
@@ -275,8 +274,12 @@ func NewDirect(opts Options) (*Direct, error) {
 	var interceptedDial *atomic.Bool
 	if httpc == nil {
 		tr := http.DefaultTransport.(*http.Transport).Clone()
-		tr.Proxy = tshttpproxy.ProxyFromEnvironment
-		tshttpproxy.SetTransportGetProxyConnectHeader(tr)
+		if buildfeatures.HasUseProxy {
+			tr.Proxy = feature.HookProxyFromEnvironment.GetOrNil()
+			if f, ok := feature.HookProxySetTransportGetProxyConnectHeader.GetOk(); ok {
+				f(tr)
+			}
+		}
 		tr.TLSClientConfig = tlsdial.Config(opts.HealthTracker, tr.TLSClientConfig)
 		var dialFunc netx.DialFunc
 		dialFunc, interceptedDial = makeScreenTimeDetectingDialFunc(opts.Dialer.SystemDial)