new item key: number of open file descriptors

A new agent item key is sought that returns the number of currently open file descriptors.
There could also be a parameter to gather this per user.

This has been discussed in https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/21170-linux-monitoring-active-open-file-descriptors .

ZBXNEXT-4323 asks for a way to monitor open FDs for deleted files, which might be an option for a generic item.