It helps to prevent a hangup with some FTP servers in case idle session
timeout has exceeded. But it may be useful also for other protocols
that send any quit message on disconnect. Currently used by FTP, POP3,
IMAP and SMTP.
If you pass a URL to pop3 that does not contain a message ID as
part of the URL, it will currently ask for 'INBOX' which just
causes the pop3 server to return an error.
The change makes libcurl treat en empty message ID as a request
for LIST (list of pop3 message IDs). User's code could then
parse this and download individual messages as desired.