feature/*: make Wake-on-LAN conditional, start supporting modular features

This pulls out the Wake-on-LAN (WoL) code out into its own package
(feature/wakeonlan) that registers itself with various new hooks
around tailscaled.

Then a new build tag (ts_omit_wakeonlan) causes the package to not
even be linked in the binary.

Ohter new packages include:

   * feature: to just record which features are loaded. Future:
     dependencies between features.
   * feature/condregister: the package with all the build tags
     that tailscaled, tsnet, and the Tailscale Xcode project
     extension can empty (underscore) import to load features
     as a function of the defined build tags.

Future commits will move of our "ts_omit_foo" build tags into this
style.

Updates #12614

Change-Id: I9c5378dafb1113b62b816aabef02714db3fc9c4a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/feature.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package feature tracks which features are linked into the binary.
+package feature
+
+var in = map[string]bool{}
+
+// Register notes that the named feature is linked into the binary.
+func Register(name string) {
+	if _, ok := in[name]; ok {
+		panic("duplicate feature registration for " + name)
+	}
+	in[name] = true
+}