Fix race condition with unix process listing

[aelsabbahy]

When a system is aggressively launching new processes in parallel, it seems the processes() method returns an incomplete set of results.

I was seeing the issue in goss, here: goss-org/goss#267

I was able to resolve the issue by changing the directory lookup logic from:

for {
    fis, err := d.Readdir(10)
    if err == io.EOF {
        break
    }
}

To just simply:

fis, err := d.Readdir(-1)

I assume the size of /proc shouldn't cause concern with the -1 full read.