[Dovecot] Making POP3 not retrieve deleted messages

Since my users often use the webmail system to delete messages to save their poor strained dial-up POP3-using mail clients, and our new web mail system isn't expunge-happy like the last one, I found need for this patch -- it adds a configuration option for pop3 "pop3_ignore_deleted", a boolean, that does just that.

I'd love feedback.

Aria