bus-proxy: don't close local bus fds twice

bus-proxy: don't close local bus fds twice

Clear up how we pass fd owner ship to proxy and bus objects. Document
that ownership is passed of the fds in question even in case of failing
constructors, and that callers should forget about fds pass into the
proxy object.

The alternative would be to duplicate the fds, but given that fds are a
relatively scarce and heavy resource let's better avoid that.

Fixes #1591.