feature/featuretags, all: add build features, use existing ones in more places

Saves 270 KB.

Updates #12614

Change-Id: I4c3fe06d32c49edb3a4bb0758a8617d83f291cf5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnauth/ipnauth_unix_creds.go

@@ -0,0 +1,36 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !windows && !ts_omit_unixsocketidentity
+
+package ipnauth
+
+import (
+	"net"
+
+	"github.com/tailscale/peercred"
+	"tailscale.com/types/logger"
+)
+
+// GetConnIdentity extracts the identity information from the connection
+// based on the user who owns the other end of the connection.
+// and couldn't. The returned connIdentity has NotWindows set to true.
+func GetConnIdentity(_ logger.Logf, c net.Conn) (ci *ConnIdentity, err error) {
+	ci = &ConnIdentity{conn: c, notWindows: true}
+	_, ci.isUnixSock = c.(*net.UnixConn)
+	if ci.creds, err = peercred.Get(c); ci.creds != nil {
+		ci.pid, _ = ci.creds.PID()
+	} else if err == peercred.ErrNotImplemented {
+		// peercred.Get is not implemented on this OS (such as OpenBSD)
+		// Just leave creds as nil, as documented.
+	} else if err != nil {
+		return nil, err
+	}
+	return ci, nil
+}
+
+// WindowsToken is unsupported when GOOS != windows and always returns
+// ErrNotImplemented.
+func (ci *ConnIdentity) WindowsToken() (WindowsToken, error) {
+	return nil, ErrNotImplemented
+}