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
It would make sense if the compiler warned on this line, but it's not the case here. And generally we try to avoid casts whenever possible (not always possible), and silence specific cases of warnings only when absolutely necessary.