imap: Re-factored final IMAP_AUTHENTICATE constant

Changed the final IMAP_AUTHENTICATE constant to IMAP_AUTHENTICATE_FINAL
for consistency with the response function.
pop3: Re-factored the final POP3_AUTH constant

Changed the final POP3_AUTH constant to POP3_AUTH_FINAL for consistency
with the response function.
smtp: Re-factored the final SMTP_AUTH constant

Changed the final SMTP_AUTH constant to SMTP_AUTH_FINAL for consistency
with the response function.