control,ipn: add Tailnet hint for device registration

Add an ipn.Prefs.Tailnet field, passed through controlclient.Options
into tailcfg.RegisterRequest.Tailnet, so a client can recommend or
require a tailnet at registration (e.g. to skip the tailnet picker when
the intent is already known) without impersonating MDM.

Previously this value could only come from the "Tailnet" syspolicy
setting. That setting still takes precedence as an administrative
override; the prefs value is used only when no policy is set.

Updates #22093

Signed-off-by: Adriano Sela Aviles <adriano@tailscale.com>
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