POP/LMTP/TIMSIEVE: re-enable slow client support
fix simple cmmi run, dburi support, sqlite autocreate

running a straightforward 'configure; make; make install' will now
try to install the default dbmail.conf

dbmail.conf now supports a new dburi parameter which will enable SSL
database client support if enabled in libzdb. The old database
parameters are now deprecated.

if using sqlite, if the database looks empty (no dbmail_users table) a
fresh database scheme will be installed.