Ticket 342: Have READ-BYTEVECTOR(!) return 0 at EOF

Currently, read-bytevector and read-bytevector! return an EOF object at EOF; otherwise, the number of bytes read is returned. Returning 0 at EOF instead would make the result always an exact integer. The only ambiguity would be that reading a bytevector of length 0 (which is to say, not reading any bytes at all) would not report EOF; however, this is a pointless thing to do.