Not all aggregation functions work with foreach functions

Steps to reproduce:

1. Try to create an itemprototype like: varpop(avg_foreach(/*/some.zabbix.key{#LLDMACRO}?group="{#HOSTGROUPLLD}",1d))

Result:
The web frontend will tell you incorrect usage for function 'varpop'. The same is true for varsamp, stddevsamp and stddevpop

Expected:
This should work according to the documentation since the change in trigger and aggregate item syntax into the new unified syntax. 

It also did not work in version 5.4.10.

What's interesting is you can successfully test the item with the Test button when you go through the "Create Item" display. So, the calculation works, but something is incorrectly invalidating the syntax.

We would really like this patched in 6.0 because a calculation like the one above is incredibly valuable when looking across a cluster of related hosts.