ipnlocal,proxymap,wgengine/netstack: add optional WhoIs/proxymap debug

Updates tailscale/corp#20600

Change-Id: I2bb17af0f40603ada1ba4cecc087443e00f9392a
Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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