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
When interfacing with APIs that expect unix-style async I/O, it is
useful to be able to tell in advance whether a read/write is going to
block. This adds new interfaces GPollableInputStream and
GPollableOutputStream that can be implemented by a GInputStream or
GOutputStream to add _is_readable/_is_writable, _create_source, and
_read_nonblocking/_write_nonblocking methods.
Also, implement for GUnixInput/OutputStream and
GSocketInput/OutputStream
https://bugzilla.gnome.org/show_bug.cgi?id=634241
