When reverse path filtering is in strict mode on Linux, using an exit
node blocks all network connectivity. This change adds a warning about
this to `tailscale status` and the logs.
Example in `tailscale status`:
```
- not connected to home DERP region 22
- The following issues on your machine will likely make usage of exit nodes impossible: [interface "eth0" has strict reverse-path filtering enabled], please set rp_filter=2 instead of rp_filter=1; see https://github.com/tailscale/tailscale/issues/3310
```
Example in the logs:
```
2024/02/21 21:17:07 health("overall"): error: multiple errors:
not in map poll
The following issues on your machine will likely make usage of exit nodes impossible: [interface "eth0" has strict reverse-path filtering enabled], please set rp_filter=2 instead of rp_filter=1; see https://github.com/tailscale/tailscale/issues/3310
```
Updates #3310
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
8cc5c51888