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>