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
So I had used CF_INFINITY because the user can specify a size to read_file. Ideally, I'd like this to match the other cases where a user can specify a size.

IMO we should probably have a *very* large upper limit for reading files like these. I didn't add one in this diff, since it probably requires some discussion. CF_INFINITY happened to work well here, since users can specify inf as a possible size.