pop3: Get message listing if no mailbox in URL

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.

lib/pop3.h

@@ -32,6 +32,7 @@ typedef enum {
   POP3_USER,
   POP3_PASS,
   POP3_STARTTLS,
+  POP3_LIST,
   POP3_RETR,
   POP3_QUIT,
   POP3_LAST  /* never used */