make readFile accept fd

[rlidwka]

We have an fd of the file that's already opened, and need to read the entire file.

I suppose I could just copy fs.readFile and remove fs.open from it, but it'd be nicer if fs.readFile would just accept an fd.

[chrisdickinson]

I imagine a PR for this would be welcome!

[jwueller]

I have contributed before and am interested in doing this one to get back in there. I'll check it out tomorrow.