redmine #2912: readfile and textxform functions do not rely on 4k limit anymore

This is a PoC for discussion on the dev-cfengine list.

- Change allocation in `FnCallTextXform` to be dynamic instead of hammering the stack
- Change limit in `FnCallTextXform` and `FnCallReadFile` to CF_INFINITY

https://dev.cfengine.com/issues/2912
Small detail but for the sake of completion, If the user passes "inf" to the function, then this function shouldn't even check how big the file is, no upper limit. I didn't check if this is possible with our CfReadfile backend function though.