gio: minor GPollableInputStream / GPollableOutputStream fixes

gio: minor GPollableInputStream / GPollableOutputStream fixes

Make g_pollable_input_stream_read() and
g_pollable_output_stream_write() look a little bit more like the
non-pollable versions in terms of error handling, etc. Also, use the
read_fn and write_fn virtual methods directly rather than calling
g_input_stream_read()/g_output_stream_write(), to avoid problems with
re-entrancy involving the "pending" flag.

Also belatedly add single-include guards to the header files.

https://bugzilla.gnome.org/show_bug.cgi?id=673997