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>

cmd/tailscale/cli/cli.go

@@ -170,6 +170,7 @@ change in the future.
 			ipCmd,
 			statusCmd,
 			pingCmd,
+			ncCmd,
 			versionCmd,
 			webCmd,
 			fileCmd,

cmd/tailscale/depaware.txt

@@ -56,6 +56,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/net/neterror                                   from tailscale.com/net/netcheck+
         tailscale.com/net/netknob                                    from tailscale.com/net/netns
         tailscale.com/net/netns                                      from tailscale.com/derp/derphttp+
+        tailscale.com/net/netutil                                    from tailscale.com/client/tailscale
         tailscale.com/net/packet                                     from tailscale.com/wgengine/filter
         tailscale.com/net/portmapper                                 from tailscale.com/net/netcheck+
         tailscale.com/net/stun                                       from tailscale.com/net/netcheck