honor the client Logger

[ainghazal]

in minivpn/vpn, at some point I made an attempt at passing a logger from options to the client.
that attempt is not completed (and Options should perhaps not be polluted with this); the interface is there, but right now all the logs are using the default (global) logger.

should either normalize use of client's logger (which means passing it down the hierarchy)... or perhaps setup the global logger at the beginning of Start() based on any non-nil implementation in Client.

[ainghazal]

related to this: extras could ship a convenience nullLogger implementation.