Dovecot configuration changes

[kiekerjan]

Two small configuration changes for dovecot:

• Calculate the default_client_limit to be in line with the default_process_limit that is set. Dovecot uses the process_limit multiple times to calculate the client_limit. This change prevents the following warning in the dovecot log: config: Warning: service auth { client_limit=1250 } is lower than required under max. load (2500)
• Set the log_path to the default. This means logging will run via syslog instead of directly into the mail.log file. This prevents issues e.g. when logrotate moves mail.log to mail.log.1, dovecot will continue to log to its file reference, that is, it continues to log to mail.log.1, until it is restarted by the daily backup process.