Fix proxy multiplexing (-O proxy) bug

Fix proxy multiplexing (-O proxy) bug

If a mux started with ControlPersist then later has a forwarding added using
mux proxy connection and the forwarding was used, then when the mux proxy
session terminates, the mux master process will send a channel close to the
server with a bad channel ID and crash the connection.

This was caused by my stupidly reusing c->remote_id for mux channel
associations when I should have just added another member to struct channel.

ok markus@