Add verbose logging flag to log incoming connections

[bradfitz]

In server mode, there should be a flag to log all incoming connections: from what IP/key, to what key/service, etc.

Options:

• --logs
• --verbose
• --server-logs
• --log-clients

etc

[bradfitz]

Oh, there's already --verbose. But that's just a bunch of debug spam. Probably not the best place to put it.