We already present a health warning about this, but it is easy to miss
on a server when blackholing traffic makes it unreachable.
In addition to a health warning, present a risk message when exit node
is enabled.
Example:
```
$ tailscale up --exit-node=lizard
The following issues on your machine will likely make usage of exit nodes impossible:
- interface "ens4" has strict reverse-path filtering enabled
- interface "tailscale0" has strict reverse-path filtering enabled
Please set rp_filter=2 instead of rp_filter=1; see https://github.com/tailscale/tailscale/issues/3310
To skip this warning, use --accept-risk=linux-strict-rp-filter
$
```
Updates #3310
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
db34cdcfe7