cmd/tailscale, ipn/ipnlocal: add "peerapi" ping type

For debugging when stuff like #4750 isn't working.

RELNOTE=tailscale ping -peerapi

Change-Id: I9c52c90fb046e3ab7d2b121387073319fbf27b99
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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