search for: nbd_unlocked_set_export_nam

I'm not sure whether we want to go with just the first patch (reject nbd:unix:/path but still accept nbd:/path), or squash the two in order to go with the second (reject both abbreviated forms, and require scheme://...). Either way, though, nbdkit -U - --run '$nbd' will now error out rather than inadvertently connect over TCP to localhost:10809 instead of the intended Unix connection

This isn't quite finished because not all of the tests or examples have been updated, but it demonstrates an idea: Should we forget about the concept of having multiple connections managed under a single handle? In this patch there is a single ‘struct nbd_handle *’ which manages a single state machine and connection (and therefore no nbd_connection). To connect to a multi-conn server you must