Updates tailscale/tailscale#3363
Updates tailscale/tailscale#6148
Provides a facility for the iOS code (and later Android) to signal the beginning of device-wide sleep mode to the LocalBackend, and wires it up to the DNS forwarder, to early-close any open DoH connections when the device is about to enter sleep mode (we don't want a single TCP keepalive to wake up the device again seconds later).
Signed-off-by: Andrea Gottardo <andrea@gottardo.me>