Trend computing function

In the attachment is a dirty hack that adds support for trend computation based on function evaluate_SUM. The function does linear regression on the data an returns the trend of the fitted function.

Right now I'm using it to predict when the space on disk will fill:

1) zabbix agent item: vfs.fs.size[/,pfree]
2) calculated item that computes time to live in hours named "vfs.fs.size[/,ttl]":
-last("vfs.fs.size[/,pfree]")/(trend("vfs.fs.size[/,pfree]",21600)*3600-0.000001)
3) trigger
{vfs.fs.size[/,ttl].last(#1)}>0&{vfs.fs.size[/,ttl].last(#1)}<48

Specification https://www.zabbix.org/wiki/Docs/specs/ZBXNEXT-922