[Dovecot] verbose imap logging

Implements more verbose imap logging (when client exits). Similar to existing logging in POP3.

Due to the nature of imap and my incomplete understanding (of the dovecot code) the session statistics reported have varying accuracy.

Much better than nothing IMO.

Cheers, Jens

 'This mail automatically becomes portable when carried.'

 Jens Låås                              Email: jens.laas@data.slu.se
 Department of Computer Services, SLU   Phone: +46 18 67 35 15
 Vindbrovägen 1
 P.O. Box 7079
 S-750 07 Uppsala
 SWEDEN