readfile and textxform functions do not rely on 4k limit anymore

From kacfengine/core-4k-limit
I'm trying to read through the [documentation for readfile](https://docs.cfengine.com/docs/3.6/reference-functions-readfile.html). It doesn't mention that "0" is equivalent to "inf". It mentions a weird note about files in ```/proc``` that must be read using "0" maxbytes.

However I'm thinking: if it's not explicitly stated that 0 should mean infinite, then maybe the proper behaviour would be to really read zero bytes, i.e. always generate an empty string.

Anyway I guess that for not breaking existing scripts, we would have to keep it like it is and it should mean infinite. So @worr I guess go on and code it like that, and we'll update the documentation when it's merged. @kacfengine @estenberg @tzz what do you think? :-)