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>

feature/condregister/condregister.go

@@ -6,9 +6,13 @@
 // to ensure all conditional features are registered.
 package condregister
 
-// Portmapper is special in that the CLI also needs to link it in,
-// so it's pulled out into its own package, rather than using a maybe_*.go
-// file in condregister.
 import (
+	// Portmapper is special in that the CLI also needs to link it in,
+	// so it's pulled out into its own package, rather than using a maybe_*.go
+	// file in condregister.
 	_ "tailscale.com/feature/condregister/portmapper"
+
+	// HTTP proxy support is also needed by the CLI, and tsnet, so it's its
+	// own package too.
+	_ "tailscale.com/feature/condregister/useproxy"
 )

feature/condregister/useproxy/doc.go

@@ -0,0 +1,6 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package useproxy registers support for using proxies
+// if it's not disabled via the ts_omit_useproxy build tag.
+package useproxy

feature/condregister/useproxy/useproxy.go

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