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 feel the CF_INFINITY check here should exist at all. It's a string already in CFEngine's memory, not something from the disk or network, so bloating the memory at this point shouldn't be of a concern.

If and how we should bother for bloating the memory is a valid concern generally, it just does not belong here.