Improve the 'node/%/addanother' menu callback

The attached patch addresses 3 issues around the node/%/addanother callback.

1. We can make use of menu wildcard loaders, in this case %node (which called node_load for us). This also handles if a "bad" node is passed in for us for free (returns a 404 for us).
2. The goto function can be simplified now it received a node object only. It never seems to be used without node object before.
3. The access function has been tightened up. There was no check on node_access('update'... so you could potentially, as a non-authorised user, end up getting redirected to a node/add/{type} page by randomly hitting node ID's. Not a critical issue but could be considered information disclosure.

I've tested the patch locally and it seems to work ok - would appreciate feedback/testing from others though :)