cmd/tailscale/cli, client/tailscale, ipn/localapi: add tailscale syspolicy {list,reload} commands

In this PR, we add the tailscale syspolicy command with two subcommands: list, which displays
policy settings, and reload, which forces a reload of those settings. We also update the LocalAPI
and LocalClient to facilitate these additions.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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