ipn/ipnlocal: use an in-memory TKA store if FS is unavailable

This requires making the internals of LocalBackend a bit more generic,
and implementing the `tka.CompactableChonk` interface for `tka.Mem`.

Signed-off-by: Alex Chan <alexc@tailscale.com>

Updates https://github.com/tailscale/corp/issues/33599

cmd/tailscale/cli/up.go

@@ -818,6 +818,7 @@ func upWorthyWarning(s string) bool {
 		strings.Contains(s, healthmsg.WarnAcceptRoutesOff) ||
 		strings.Contains(s, healthmsg.LockedOut) ||
 		strings.Contains(s, healthmsg.WarnExitNodeUsage) ||
+		strings.Contains(s, healthmsg.InMemoryTailnetLockState) ||
 		strings.Contains(strings.ToLower(s), "update available: ")
 }