Login process creation

login_processes_count and login_max_processes_count setting control how new login processes are created. login_processes_count specifies the number of login processes that are tried to be kept listening for new connections. However when a lot of connections arrive at the same
time this number will increase automatically as described below.

To prevent fork-bombing Dovecot's login process creation works in a similiar way to Apache: Initially set "wanted listening process count" to same as login_processes_count and start the processes. Then check every second if we need to start up new processes to keep the listening
process count the same as the wanted count. If all the listening processes have been used, double the wanted count. If we haven't used all of them, decrease the wanted count by one (unless it goes below login_processes_count), but don't destroy already created processes.

login_max_processes_count specifies the maximum number of login processes there can exist at a time (listening, non-listening and SSL/TLS-proxying processes combined).