tailcfg: remove most Debug fields, move bulk to nodeAttrs [capver 70]

Now a nodeAttr: ForceBackgroundSTUN, DERPRoute, TrimWGConfig,
DisableSubnetsIfPAC, DisableUPnP.

Kept support for, but also now a NodeAttr: RandomizeClientPort.

Removed: SetForceBackgroundSTUN, SetRandomizeClientPort (both never
used, sadly... never got around to them. But nodeAttrs are better
anyway), EnableSilentDisco (will be a nodeAttr later when that effort
resumes).

Updates #8923

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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