ipn/ipnserver paths: add paths.LegacyStateFilePath

Moving this information into a centralized place so that it is accessible to
code in subsequent commits.

Updates #3011

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

paths/paths_windows.go

@@ -148,3 +148,9 @@ func ensureStateDirPerms(dirPath string) error {
 	return windows.SetNamedSecurityInfo(dirPath, windows.SE_FILE_OBJECT, flags,
 		sids.User, sids.PrimaryGroup, dacl, nil)
 }
+
+// LegacyStateFilePath returns the legacy path to the state file when it was stored under the
+// current user's %LocalAppData%.
+func LegacyStateFilePath() string {
+	return filepath.Join(os.Getenv("LocalAppData"), "Tailscale", "server-state.conf")
+}