netConn and msgReader should return io.EOF identically instead of wrapped

[spikecurtis]

*bufio.Scanner and other consumers of io.Reader interface expect to receive io.EOF identically when reading to the end of a file/stream.

From the io package:

// EOF is the error returned by Read when no more input is available.
// (Read must return EOF itself, not an error wrapping EOF,
// because callers will test for EOF using ==.)
// Functions should return EOF only to signal a graceful end of input.
// If the EOF occurs unexpectedly in a structured data stream,
// the appropriate error is either [ErrUnexpectedEOF] or some other error
// giving more detail.
var EOF = errors.New("EOF")

We currently wrap EOF along with any other error when reading from netConn or msgReader. Instead, we should return io.EOF identically to conform with the io.Reader spec.