client/tailscale: move/copy all package funcs to new LocalClient type

Remove all global variables, and clean up tsnet and cmd/tailscale's usage.

This is in prep for using this package for the web API too (it has the
best package name).

RELNOTE=tailscale.com/client/tailscale package refactored w/ LocalClient type

Change-Id: Iba9f162fff0c520a09d1d4bd8862f5c5acc9d7cd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f