client: Added an EOF flag to the client object.

client: Added an EOF flag to the client object.

In a lot of cases, a client is passed through a couple of functions in order
to handle I/O. Adding the flag will make it easier to access the information
in the right location without having to worry about 0 return codes (which mean
different things in some cases anyway, e.g. in sdb_client_recv() which returns
zero if an empty message (header only) has been received).