Don't use separate asynchronous event connection for user datapath.

Don't use separate asynchronous event connection for user datapath.

Commit 14439fa80c, "Maintain separate async and sync connections to nl:0
in secchan," modified secchan to use two separate connections to the
datapath, one for asynchronous events, one for requests and replies.  This
technique doesn't work for the user datapath, which always sends
asynchronous events on all its connections.  Fortunately, it isn't
necessary for the user datapath, either, because the user datapath is
smart enough not to drop message replies.

Tested by Justin.