ipn/ipnlocal: change serial number policy to be PreferenceOption

This commit changes the PostureChecking syspolicy key to be a
PreferenceOption(user-defined, always, never) instead of Bool.

This aligns better with the defaults implementation on macOS allowing
CLI arguments to be read when user-defined or no defaults is set.

Updates #tailscale/tailscale/5902

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
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