renamed from `useNetstack` to `onlyNetstack` which is 1 letter more but
more descriptive because we always have netstack enabled and `useNetstack`
doesn't convey what it is supposed to be used for. e.g. we always use
netstack for Tailscale SSH.
Also renamed shouldWrapNetstack to handleSubnetsInNetstack as it was only used
to configure subnet routing via netstack.
Updates tailscale/corp#8020
Signed-off-by: Maisem Ali <maisem@tailscale.com>