pop3: Changed response code from O and E to + and -

The POP3 protocol doesn't really have the concept of error codes and
uses +, +OK and -ERR in response to commands to indicate continue,
success and error.

The AUTH command is one of those commands that requires multiple pieces
of data to be sent to the server where the server will respond with + as
part of the handshaking. This meant changing the values before
continuing with the next stage of adding authentication support.