Fix error checking in writing files

Fix error checking in writing files

For write, 0 may not mean an error at all. We need to instead check for the length not being the same.

With fwrite, because 0 could mean an error, but not always. We must check that we wrote the entire file!

Note that unlike write, fwrite's description according to POSIX does not mention returning a negative type at all. Nor does it say you can retry unlike write.

Finally, with write, we need to check less than 0, not 0, as 0 is a valid return and does not mean an error.