firewall.py: make it super clear when we apply the MacOS fix.
firewall.py: make it super clear when we apply the MacOS fix.

Print a message to stderr, then abort.  But only the first time.

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