paths: remove wasm file, no-op stubs, make OS-specific funcs consistent

Some OS-specific funcs were defined in init. Another used build tags
and required all other OSes to stub it out. Another one could just be in
the portable file.

Simplify it a bit, removing a file and some stubs in the process.

Updates #5794

Change-Id: I51df8772cc60a9335ac4c1dc0ab59b8a0d236961
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

paths/paths.go

@@ -45,7 +45,14 @@ func DefaultTailscaledSocket() string {
 	return "tailscaled.sock"
 }
 
-var stateFileFunc func() string
+// Overridden in init by OS-specific files.
+var (
+	stateFileFunc func() string
+
+	// ensureStateDirPerms applies a restrictive ACL/chmod
+	// to the provided directory.
+	ensureStateDirPerms = func(string) error { return nil }
+)
 
 // DefaultTailscaledStateFile returns the default path to the
 // tailscaled state file, or the empty string if there's no reasonable
@@ -67,6 +74,16 @@ func MkStateDir(dirPath string) error {
 	if err := os.MkdirAll(dirPath, 0700); err != nil {
 		return err
 	}
-
 	return ensureStateDirPerms(dirPath)
 }
+
+// LegacyStateFilePath returns the legacy path to the state file when
+// it was stored under the current user's %LocalAppData%.
+//
+// It is only called on Windows.
+func LegacyStateFilePath() string {
+	if runtime.GOOS == "windows" {
+		return filepath.Join(os.Getenv("LocalAppData"), "Tailscale", "server-state.conf")
+	}
+	return ""
+}