Little things in RemoteVstPlugin behaviour

`RemoteVstPlugin::load()` assigns itself to `AEffect.user` but never does anything with that field.

I wrote a little test plugin and ran it in several hosts, and here are the test results:
https://gist.github.com/grejppi/9178616

It seems `ptr1` and `ptr2` are for the host to use; no other host touched `user` except LMMS. And LMMS doesn't even need it...

Also, I guess the host should deactivate the plugin properly, by sending `effMainsChanged`...