net/tstun: move TAP support out to separate package feature/tap

Still behind the same ts_omit_tap build tag.

See #14738 for background on the pattern.

Updates #12614

Change-Id: I03fb3d2bf137111e727415bd8e713d8568156ecc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/feature.go

@@ -4,6 +4,8 @@
 // Package feature tracks which features are linked into the binary.
 package feature
 
+import "reflect"
+
 var in = map[string]bool{}
 
 // Register notes that the named feature is linked into the binary.
@@ -13,3 +15,40 @@ func Register(name string) {
 	}
 	in[name] = true
 }
+
+// Hook is a func that can only be set once.
+//
+// It is not safe for concurrent use.
+type Hook[Func any] struct {
+	f  Func
+	ok bool
+}
+
+// IsSet reports whether the hook has been set.
+func (h *Hook[Func]) IsSet() bool {
+	return h.ok
+}
+
+// Set sets the hook function, panicking if it's already been set
+// or f is the zero value.
+//
+// It's meant to be called in init.
+func (h *Hook[Func]) Set(f Func) {
+	if h.ok {
+		panic("Set on already-set feature hook")
+	}
+	if reflect.ValueOf(f).IsZero() {
+		panic("Set with zero value")
+	}
+	h.f = f
+	h.ok = true
+}
+
+// Get returns the hook function, or panics if it hasn't been set.
+// Use IsSet to check if it's been set.
+func (h *Hook[Func]) Get() Func {
+	if !h.ok {
+		panic("Get on unset feature hook, without IsSet")
+	}
+	return h.f
+}