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
Dear Dimitrios,

>>      char *string = RlistScalarValue(finalargs);
>> -    int len = 0;
>> +    FnCallResult ret;
>> +    size_t len = strlen(string);
>> +	size_t bufsiz = len + 1 < 2 ? 2 : len + 1;

Can you please explain to me the relationship between 2612 and issue 2562
on the 2562 limit?

Met vriendelijke groet,
Martin.

+31651567029