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/buildfeatures/feature_outboundproxy_disabled.go

@@ -7,7 +7,7 @@
 
 package buildfeatures
 
-// HasOutboundProxy is whether the binary was built with support for modular feature "Outbound localhost HTTP/SOCK5 proxy support".
+// HasOutboundProxy is whether the binary was built with support for modular feature "Support running an outbound localhost HTTP/SOCK5 proxy support that sends traffic over Tailscale".
 // Specifically, it's whether the binary was NOT built with the "ts_omit_outboundproxy" build tag.
 // It's a const so it can be used for dead code elimination.
 const HasOutboundProxy = false

feature/buildfeatures/feature_outboundproxy_enabled.go

@@ -7,7 +7,7 @@
 
 package buildfeatures
 
-// HasOutboundProxy is whether the binary was built with support for modular feature "Outbound localhost HTTP/SOCK5 proxy support".
+// HasOutboundProxy is whether the binary was built with support for modular feature "Support running an outbound localhost HTTP/SOCK5 proxy support that sends traffic over Tailscale".
 // Specifically, it's whether the binary was NOT built with the "ts_omit_outboundproxy" build tag.
 // It's a const so it can be used for dead code elimination.
 const HasOutboundProxy = true

feature/buildfeatures/feature_synology_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_synology
+
+package buildfeatures
+
+// HasSynology is whether the binary was built with support for modular feature "Synology NAS integration (applies to Linux builds only)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_synology" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasSynology = false

feature/buildfeatures/feature_synology_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_synology
+
+package buildfeatures
+
+// HasSynology is whether the binary was built with support for modular feature "Synology NAS integration (applies to Linux builds only)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_synology" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasSynology = true

feature/buildfeatures/feature_useproxy_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_useproxy
+
+package buildfeatures
+
+// HasUseProxy is whether the binary was built with support for modular feature "Support using system proxies as specified by env vars or the system configuration to reach Tailscale servers.".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_useproxy" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasUseProxy = false

feature/buildfeatures/feature_useproxy_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_useproxy
+
+package buildfeatures
+
+// HasUseProxy is whether the binary was built with support for modular feature "Support using system proxies as specified by env vars or the system configuration to reach Tailscale servers.".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_useproxy" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasUseProxy = true