Clarified progress parameter names for the MailProtocol.
pop3: Fixed maintaining fSizes list, and more.

* The list was filled, but never emptied.
* If SyncMessages() was called more than once for the same POP3 instance,
  this could garble the mails retrieved in the second run.
* Maintain the total size of mails in fSizes to be able to report progress
  in more detail.
* Also adapted progress reporting to the argument changes made in MailProtocol
  earlier.
* Minor cleanup.
pop3: minor cleanup.