reuseport is a hack. It is necessary for us to do certain kinds of
tcp nat traversal. Ideally, reuseport would be available in go:

  https://github.com/golang/go/issues/9661

But until that issue is fixed, we're stuck with this. In some cases,
reuseport is strictly a detriment: nodes are not NATed. This commit
introduces an ENV var IPFS_REUSEPORT that can be set to false to
avoid using reuseport entirely:

  IPFS_REUSEPORT=false ipfs daemon

This approach addresses our current need. It could become a config
var if necessary. If reuseport continues to give problems, we should
look into improving it.