client/tailscale, cmd/tailscale, localapi: add 'tailscale nc'

This adds a "tailscale nc" command that acts a bit like "nc", but
dials out via tailscaled via localapi.

This is a step towards a "tailscale ssh", as we'll use "tailscale nc"
as a ProxyCommand for in some cases (notably in userspace mode).

But this is also just useful for debugging & scripting.

Updates #3802

RELNOTE=tailscale nc

Change-Id: Ia5c37af2d51dd0259d5833d80264d3ad5f68446a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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