Fix usages of fread and fwrite

[ej-uses-git]

From man 3 fread:

The function fread() does not distinguish between end-of-file and error; callers must use feof(3) and ferror(3) to determine which occurred.
The function fwrite() returns a value less than nitems only if a write error has occurred.

So we don't need to loop and check for fwrite, since it only returns anything smaller than size if it failed. It also doesn't set ferror so that's the wrong way to test for an error.

Also, while man says nothing about errno, the UNIX spec for fread and fwrite says:

Otherwise, if a read error occurs, the error indicator for the stream is set and errno is set to indicate the error.

(See here)

So I removed the TODO comment about it.