This patch guarantees that g_output_stream_write() can never fail with
G_IO_ERROR_WOULD_BLOCK. Without such a guarantee, we would need some
kind of GIOPollable interface or some way to get an event when the
stream is writable again. Which is mostly useless considering that
this method is asynchronous anyway.

Note: this patch just codifies existing behavior - GUnixOutputStream,
GSocketOutputStream and other implementations already work this way.

See also bug 626748 comment 5 for how the GDBus code relies on this
guarantee.

https://bugzilla.gnome.org/show_bug.cgi?id=627071



Signed-off-by: David Zeuthen <davidz@redhat.com>