Depending on the distro the maximum number of user processes might be determined at boot by the kernel and applied to PID1 from what all other processes inherit or it might be severly restricted globally via limits.conf like it's the case with CentOS 6 that tries to prevent accidental fork bombs.

So we introduce
- a new property NPROC in /opt/open-xchange/etc/ox-scriptconf.sh that defaults to 65536 for system v based distros
- LimitNPROC to 65536 in the default open-xchange.service for systemd based distros. This default can be overridden via our existing drop-in config at /etc/systemd/system/open-xchange.service.d/limits.conf