With netmap caching, the home DERP of the self node was neither saved to
the cache or loaded from it, making nodes not stick to a DERP when
starting without a connection to control.
Instead, make sure that when a cache is available, load that cache,
before looking for DERP servers. This is implemented by allowing a skip
of ReSTUN in setting the DERP map (we must have a DERP map before
setting the home DERP), so the DERP from cache will set itself and be
sticky until a connection to control is established.
Making DERP only change when connected to control is handled by existing
code from f072d017bd8241675aa946a27fc1827f570435cb.
Updates #19490
Signed-off-by: Claus Lensbøl <claus@tailscale.com>
