Errors inside stream write operations are currently ignored and will typically result from a 0 return value from fwrite(), contrary to documentation. This can to severe bugs, including denial of service (https://bugs.php.net/bug.php?id=73535).

This PR changes the stream APIs to use a ssize_t return value, where negative returns indicate an error, similar to the write() libc function. This is then propagated through various stream wrappers and using functions. There's probably more places that can check for errors, but this puts the basic scaffolding in place.

Ideally something similar would also be done for read operations.