Fix infinite loop when fopencookie custom write returns 0 on error

Fix infinite loop when fopencookie custom write returns 0 on error

The man page for fopencookie prescribes that custom write functions
should return 0 on error (and should definitely not return a negative
value) [1].

However, the uClibc implementation expects a negative return value in
case of an error (libc/stdio/_WRITE.c). If the write function returns 0
on error, we drop into an infinite loop if the error persists.

This patch wraps the user supplied write function such that a 0 return
value is converted to -1. errno is first set to EAGAIN such that if the
custom write function does not set errno in case of error, this is
treated as a "soft" error.

Custom write functions that cater towards uClibc and _do_ return a
negative value are not affected.

If no custom write function is supplied, set errno to EINVAL such that
this condition is treated as a "hard" error. Previously the behaviour
depended on whether the last error before the write happened to be a "hard"
or a "soft" error.

[1] http://git.kernel.org/cgit/docs/man-pages/man-pages.git/tree/man3/fopencookie.3?id=6c0d0ef0c7a9c4bcf805c6f9e9bc1ef1c3865ea0#n164

Signed-off-by: Jan Vangorp <jan.vangorp_ext@softathome.com>