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>

net/tshttpproxy/tshttpproxy_linux.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/url"
 
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/version/distro"
 )
 
@@ -17,7 +18,7 @@ func init() {
 }
 
 func linuxSysProxyFromEnv(req *http.Request) (*url.URL, error) {
-	if distro.Get() == distro.Synology {
+	if buildfeatures.HasSynology && distro.Get() == distro.Synology {
 		return synologyProxyFromConfigCached(req)
 	}
 	return nil, nil