Reworked the command sending from two specific LIST and RETR command
functions into a single command based function as well as the two
associated response handlers into a generic command handler.
This feature allows the user to specify and use additional POP3
commands such as UIDL and DELE via libcurl's CURLOPT_CUSTOMREQUEST or
curl's -X command line option.
Simplified the code to remove the need for a separate "LIST <msg id>"
command handler and state machine and instead use the LIST command
handler for both operations.
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.
