Don't pass the ucred to the client construct, just set the properties
in the protocol.
Use the client properties to get ucred.
Add the security label to the client properties (from SO_PEERSEC)
Pass a message around to make things more extensible later.
Keep fds per message if we ever want to write individual
messages.
Pass number of fds in the message header. We might need this to
close the fds when the proxy is gone.
