firewall: preserve permissions on /etc/hosts
firewall: preserve permissions on /etc/hosts

Pointed out by nisc on github.  If people use an unusual umask or have funny
permissions on /etc/hosts, sshuttle would screw it up.

We also use hardlinks to atomically backup the original /etc/hosts to
/etc/hosts.sbak the first time, rather than manually copying it.  Not sure
why I didn't think of that before.

luthfihm/sshuttle,bitmonk/sshuttle,GrimDerp/sshuttle,brianmay/sshuttle,GrimDerp/sshuttle,mralexgray/sshuttle,mralexgray/sshuttle,apenwarr/sshuttle,vStone/sshuttle,v2e4lisp/sshuttle,sshuttle/sshuttle,sshuttle/sshuttle,guiquanz/sshuttle,ru-faraon/sshuttle,vStone/sshuttle,fastly/sshuttle,vStone/sshuttle,zabracks/sshuttle,dushmis/sshuttle,guiquanz/sshuttle,sekka1/sshuttle,CasparLi/sshuttle,mralexgray/sshuttle,bitmonk/sshuttle,cgundogan/sshuttle,sekka1/sshuttle,GrimDerp/sshuttle,luthfihm/sshuttle,xtaran/sshuttle,dxq-git/sshuttle,brianmay/sshuttle,dxq-git/sshuttle,mheiges/sshuttle,loadlj/sshuttle,zabracks/sshuttle,luthfihm/sshuttle,fastly/sshuttle,mheiges/sshuttle,zabracks/sshuttle,guiquanz/sshuttle,dushmis/sshuttle,mheiges/sshuttle,michaelcontento/sshuttle,xtaran/sshuttle,apenwarr/sshuttle,michaelcontento/sshuttle,ru-faraon/sshuttle,brianmay/sshuttle,cgundogan/sshuttle,michaelcontento/sshuttle,shaunstanislaus/sshuttle,ru-faraon/sshuttle,CasparLi/sshuttle,v2e4lisp/sshuttle,dxq-git/sshuttle,shaunstanislaus/sshuttle,v2e4lisp/sshuttle,dushmis/sshuttle,brianmay/sshuttle,xtaran/sshuttle,cgundogan/sshuttle,loadlj/sshuttle,sekka1/sshuttle,fastly/sshuttle,shaunstanislaus/sshuttle,loadlj/sshuttle,CasparLi/sshuttle,bitmonk/sshuttle