Add client-side log indicator that an obfsbridge works

Bridge operators can be reasonably sure that obfsproxy works for them since they see the Registered server transport log message.

OTOH, clients can't be sure whether their obfsproxy works or whether they connected to a bridge with obfsproxy.

I think we should add PT information to one of the "connected to bridge" log messages.

Specifically, to:

log_notice(LD_DIR, "Learned fingerprint %s for bridge %s"

or to

log_notice(LD_DIR, "new bridge descriptor '%s' (%s): %s"

IIUC the latter will appear in the logs everytime we connect to a bridge, even if we knew the bridge fingerprint.