cmd/tailscaled: make embedded CLI run earlier, support triggering via env

Not all platforms have hardlinks, or not easily.

This lets a "tailscale" wrapper script set an environment variable
before calling tailscaled.

Updates #2233

Change-Id: I9eccc18651e56c106f336fcbbd0fd97a661d312e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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