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>

cmd/tailscaled/deps_test.go

@@ -222,3 +222,16 @@ func TestOmitGRO(t *testing.T) {
 		},
 	}.Check(t)
 }
+
+func TestOmitUseProxy(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_useproxy,ts_include_cli",
+		OnDep: func(dep string) {
+			if strings.Contains(dep, "tshttproxy") {
+				t.Errorf("unexpected dep: %q", dep)
+			}
+		},
+	}.Check(t)
+}