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
Desr William,

Please, get me out of the cc, I am seeing the posts twice now.

Thanks!

BTW, I feel you are doing a great job!

Met vriendelijke groet,
Martin.

+31651567029