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
I can see that ```strncpy()``` is preexisting here, but it's not the right function for the job. In addition the way you're calling it, ```len``` is always over the limits of string, which is confusing.\

Since we've already counted everything I would again just use ```memcpy()```.