Fixed startup crashing in POP3 server

Several fixes to opening index file which needs to be rebuilt. Before it didn't always rebuild the index but just fallbacked to using only memory, and that could have been quite slow since all mails in the mailbox were opened and parsed.

If there's nothing in mail_cache_fields, index rebuilding doesn't open the file anymore. This should make in-memory indexes a lot faster than before, but it could also mean some performance hits later. If client wants to fetch all mails from the mailbox, the same thing has to be done anyway.. This needs more thinking actually what is the optimal behaviour.

Support /etc/shadow-like file for passdb.