new item key: number of open files

zabbix has kernel.maxfiles item to find out the limit on the open files, but no item to obtain the currently open file count.

this can be found out from the /proc/sys/fs/file-nr file - the first number is the current, the last one is the max.

zabbix currently obtains max from /proc/sys/fs/file-max reading both numbers from /proc/sys/fs/file-nr might or might not be better if both items present.