clean up AttachOpts type

Primarily, there is no reason to have a pointer to a map. Furthermore,
make() can be used on AttachOpts directly.
Fix NAT problem with ports looping back to containers
stop looping remote:port from host to containers
Merge remote-tracking branch 'unclejack/137-fix-nat'
Continue cleaning up iptables rules from previous version, to avoid crashing after an upgrade
Merge remote-tracking branch 'dominikh/improve-attachopts'