Previously, if you created a GUnixInputStream or GUnixOutputStream
from a non-blocking file descriptor, it might sometimes return
G_IO_ERROR_WOULD_BLOCK from g_input_stream_read/g_output_stream_write,
which is wrong. Fix that. (Use the GPollableInput/OutputStream methods
if you want non-blocking I/O.)

Also, add a test for this to gio/tests/unix-streams.

Also, fix the GError messages to say "Error reading from file
descriptor", etc instead of "Error reading from unix" (which was
presumably from a bad search and replace job).

https://bugzilla.gnome.org/show_bug.cgi?id=626866