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
```xsnprintf()``` will abort if the allocated space is too small. I believe it can't happen hear, but to make it clear that this buffer may store a number I would change the xcalloc above to ```MAX(len+1, PRINTSIZE(len))``` bytes.