Also ignore gaierror on revdns check

This seems a little odd to me, but i don't know a lot about networking stuff.

I'd been using sshuttle successfully for several days with `--auto-hosts` and `--seed-hosts`
when all of a sudden it became unusable:
it would die after just a few seconds when it tried to lookup something that looked like an ip address (when I expected it to be looking for a name).
Like I said, it seemed odd to me, but after watching the verbose output go by and then error,
I tried this patch and it worked.

If this seems odd to you and you want more information let me know and I'll try to track down whatever was causing it again.

Otherwise, what do you think about this?

Thanks!
and thanks for this project... it's a huge help for me (instead of sshing to work, then tmux and all my other ssh's under that).