Support for connect only + listening

[davecgh]

The current approach of connect where you only connect to the specified peers also disables listening.

This make sense for most use cases, however there is also the case where a user might want to connect only to specific peers as well as listen while controlling the access via their firewall.

One such scenario is a user could setup a tor hidden service while still only connecting to trusted nodes.

Suggested by todd

[toddfries]

Glancing at the code, it would add a lot of complexity to do something
other than the following simple change.

Users can then use nolisten= as they prefer, without connect= explicitly
setting it for them.

Separately I will test to see if config options are applied in order. If that
is the case, then one simply can list 'nolisten=0' after the last 'connect='
line.

--- a/config.go +++ b/config.go @@ -351,7 +351,6 @@ func loadConfig() (*config, []string, error) { // Connect means no seeding or listening. if len(cfg.ConnectPeers) > 0 { cfg.DisableDNSSeed = true - cfg.DisableListen = true }

[davecgh]

This has been implemented via #33.