tailscaled.service: Lock down clock and /dev (#1071)

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Research in issue #1063 uncovered why tailscaled would fail with
ProtectClock enabled (it implicitly enabled DevicePolicy=closed).

This knowledge in turn also opens the door for locking down /dev
further, e.g. explicitly setting DevicePolicy=strict (instead of
closed), and making /dev private for the unit.

Additional possible future (or downstream) lockdown that can be done
is setting `PrivateDevices=true` (with `BindPaths=/dev/net/`), however,
systemd 233 or later is required for this, and tailscaled currently need
to work for systemd down to version 215.

Closes https://github.com/tailscale/tailscale/issues/1063

Signed-off-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>

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